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    My Journey through the Astronomical Year

    Think of this as a "companion text" to this, the main web site. Not required reading, butI hope you'll find it interesting and helpful.

June 2012 Events – Venus at the start, Venus at the end – and lots of cool stuff in between

Here’s a quick summary of June 2012 highlights:

All month – Mars and Saturn

Mars and Saturn put on a continuing display in the southwest – they are near equal in brightness and very close – in brightness – to the first magnitude guide stars, Regulus and Spica.

Mars, Saturn, Regulus, and Spica are the bright “stars” we see in the southwestern sky on a June evening. Can you put them in order of brightness? (Answer at the end of this post.) Click image for larger view. Prepared from Starry Nights Pro screen shot.

June 4 – Partial eclipse of the Moon

This event favors the western United States and Australia – but some parts of the eastern US can see some of it. First contact for the penumbra- the faint part of Earth’s shadow – is at  08:48:09 UT; the umbra makes contact at 09:59:53 UT, and the umbral phase is over at 09:59:53 UT. The penumbral phase ends at 13:18:17 UT.

For a map showing visibility of this event throughout the world, as well as other details, please go here.  To convert Universal Time (GMT) to your local time, please go here. GMT is selected as the default and eclipse times are in 24-hour format.

June 5-6 – Transit of  Venus

Historic  Transit of Venus – you don’t want to miss this one. Of course, you need the weather to be on your side, plus proper safety precautions for viewing the Sun. Go here for all the details.

June 20 – The Summer Solstice

The sun rises and sets as far north as it can; twilight seems to last forever; and the really dark night is darned short. Here in Westport, MA, the Sun sets at 8:22 pm EDT, but astronomical twilight doesn’t end until 10:34 pm EDT. That’s when it’s truly dark. And the morning phase of astronomical twilight begins at 02:59 am EDT. That gives us just four hours, 25 minutes of full darkness that night.

June 15 to early July – Catch the elusive planet Mercury in the evening sky – special treat June 21!

For many years I thought Mercury was really difficult to see. It isn’t, but it has that reputation for a reason. The problem with seeing Mercury is you have to know exactly when and where to look, but every year there are two or three good opportunities. This year one of them comes in the last couple of weeks of June.  During that period Mercury will be roughly 10 degrees above the horizon (one fist held at arm’s length –  to the north of west. The planet will be playing tag with the familiar Castor and Pollux – the Heavenly Twins – and will make a fine group with them and the crescent Moon on June 21.

Here are three  charts that should help you find it – but pay attention to the time. That is critical. You have to catch Mercury after sunset when the sky is dark enough for it to be seen, yet not too long after sunset or the planet will be too close to the western horizon and difficult to see through all the atmosphere. I suggest you start searching with binoculars about 15 minutes after sunset. Mercury should be visible to the naked eye around 30 minutes to an hour after sunset depending on local conditions. It’s easier to see with the naked eye once you spot it in binoculars, and it will be a bit brighter than the Twins.

All the named stars will be roughly the same brightness as Mercury, but it is hard to make comparisons in a twilight sky because the nearer to the horizon a star (or planet), the lighter the background against which you’ll see it. Click image to see a larger version. Prepared from Starry Nights Pro screen shot.

Click image to see a larger version. Prepared from Starry Nights Pro screen shot.

Click image to see a larger version. Prepared from Starry Nights Pro screen shot.

Last week of June – Jupiter and Venus vie for the title “Morning Star”

The giant planet Jupiter is back with its four bright moons and will be with us for many months to come.  It starts out the month rising just before the Sun, but by the end of the month will rise a full two hours before dawn. What’s more, it will be joined by a crescent Venus, fresh from its historic transit of the Sun at the start of the month.  And if that’s not enough, this all takes place in one of the most beautiful sections of the night sky with the charming cluster of the Pleiades hanging above the scene and the  Hyades just coming into view below it.

To see this you need an unobstructed eastern horizon; binoculars will make it much more enjoyable, and I plan to start looking about two hours before sunrise when the Pleiades are visible in a dark sky. Jupiter and Venus may not pop into view for another hour, and by then morning twilight will be getting strong – but they are so brilliant it shouldn’t matter.  However,  if you don’t see them on June 29, by all means give it a try any of several days before or after that date. From morning to morning Venus will appear to be trying to overtake Jupiter – and will get a bit closer in early July.

Capella, one of the brightest stars is near magnitude 0 – Jupiter about -2 and Venus a dazzling -4.4. Use binoculars to see the Pleiades. Click image for larger version. Prepared from Starry Nights Pro screen shot.

Answers to the brightness question

On June 15 Saturn shines at magnitude .61 and thus is the brightest, but Mars is extremely close at magnitude .7. (Extra pat on the back if you got these two correct! ) Spica is next at .98 and then Regulus at at 1.34. As the month goes on both Mars and Saturn will get a tad dimmer, but the order of brightness will remain the same – Saturn, Mars, Spica, and Regulus.

Look East in June 2012 and see if you can make the stars “pop!”

How can we make the stars pop out of the sky and into our mind’s eye? That’s the perennial problem for me, for what we actually see is so much less than what is actually there that we can’t help but to either ignore or belittle the stars unintentionally.

This month’s guide star, Deneb, is a prime example. It’s easy to spot using our chart as it rises in the northeast below and to the left of Vega. In terms of our bright guide star list, Deneb’s rather dim – 19th in the list of brightest stars we see with the naked eye. But that reveals much more about our point of view than about Deneb.

Deneb, plain and simple, is one of the most luminous stars in our galaxy. Vega, just above and to the right of it in the northeast, looks so much brighter – but it isn’t. It’s simply so much closer. Vega is just 25 light years away. Deneb, by the most recent calculations, is 1,425 light years from us. Put Deneb in Vega’s place and it would be visible in broad daylight! Does that help it “pop?”

When astronomers talk about how “luminous” a star is they don’t mean how bright it appears to us in our night sky. They mean how bright it actually is. In fact, frequently they use “luminous” to include all the radiation that comes from a star – even radiation in wavelengths that we don’t even see, such as infrared and ultraviolet. They then compare a star’s luminosity with the luminosity of the Sun – the Sun being “1.” When they examine Deneb that way they get a luminosity of 54,400 Suns – awesome! (Popping yet? Can you imagine our Sun being 54,400 times brighter?)  But when we look at Deneb we see a star that is just moderately bright – magnitude 1.25.

Click image for a larger version. The full “Summer Triangle” asterism will be easier to see as the month goes on, and the key stars get higher earlier in the evening. The key focus this month is the guide star Deneb.

For a printer-friendly version of this chart, click here.

OK – let’s get serious about this popping business. When you look at Deneb, you have to use your mind’s eye to see it for what it really is, not just for what it appears to be. So what should we see when we look at Deneb?

First we should see something huge. Deneb is classed as a “supergiant.” So sit back and try to imagine a star whose diameter is 108 times that of the Sun. No, wait! First imagine how big the Earth is. Then get in your mind the fact that the Sun is 109 times the diameter of the Earth. Got that? Now try to imagine that Deneb is to our Sun what our Sun is to the Earth.

birdshotBut wait! I really do not want to talk about diameters. Those are for people who live in a flat world. Think in terms of volume, because that’s what a planet or star really is – a volume – a mass formed into a sphere. To get your mind around volume, picture the earth as a tiny bird shot just 2.5mm in diameter. Here’s one to give you the idea.

Now picture a sphere about 10.5 inches in diameter – a basketball would be close, or this glass garden globe. See the difference? When talking diameters, the Sun is 109 Earths. But when you’re talking volume, you could fit well over a million Earths inside the Sun.

sun_deneb

Now think about the same thing in terms of Deneb. That little lead shot is our magnificent Sun. The blue globe is Deneb! That’s what you should see in your mind’s eye when you watch this month’s guide star rise in the northeast. Were Deneb our Sun, its surface would reach halfway to the orbit of Earth and needless to say, Earth would be in a hopelessly hot location.

But there’s more, of course. Size is a great starting point, but it doesn’t equate with mass. A lot of stars are bloated – that is, their mass is spread out over a large area and they have a huge surface area from which to radiate a tremendous amount of energy. That is the case with Deneb. It is believed to be about 10-15 solar masses, but its total luminosity – the total amount of energy it radiates when compared to the Sun is a whopping 54,400 times that of the Sun! Wrap your mind’s eye around that!

That’s why astronomer/author James Kaler writes that Deneb is

among the intrinsically brightest stars of its kind (that is, in its temperature or spectral class) in the Galaxy. If placed at the distance of Vega, Deneb would shine at magnitude – 7.8, 15 times more brightly than Venus at her best, be as bright as a well-developed crescent Moon, cast shadows on the ground, and easily be visible in broad daylight.

Deneb is unusual for supergiant stars for it is of spectral Class A – that means it’s your basic white star and very hot as stars go. Other very large stars, such as Betelgeuse, are in a different stage of development and quite cool and red to the eye. Deneb is believed to be just 10 million years old. That’s very young in terms of star ages. Our Sun is believed to be 5 billion years old. Deneb will never get to that ripe old age. Massive stars such as Deneb live in the fast lane, burning up their core hydrogen fuel relatively quickly.

Kaler gives this analysis:

The star is evolving and has stopped fusing hydrogen in its core. However, it’s hard to know just what is going on. It might be expanding and cooling with a dead helium core and on its way to becoming a red supergiant, or it might have advanced to the state of core helium fusion. Having begun its life as a hot class B (or even class O) star of 15 to 16 solar masses just over 10 million years ago, its fate is almost certainly to explode sometime astronomically soon as a grand supernova.

Kaler certainly knows what he’s talking about, but don’t bother to keep a “death watch” on Deneb. “Astronomically soon” means some time in the next 100 million years or so 😉

Sherlock Holmes once chided his companion Watson saying “you see, but you do not observe.” With the stars, we have to take our cue from Holmes. We have to go beyond merely seeing. And in truth, we have to go beyond merely observing. We have to take the knowledge the scientists have given us and somehow apply it to what we see, so with our mind’s eye we truly observe. Only then can we pop Deneb out of that “twinkle, twinkle little star” category and see it for what it really is.

Vital stats for Deneb (DEN-ebb), also known as Alpha Cygni:

• Brilliance: Magnitude 1.24; its luminosity is the equal of 54,400 Suns.
• Distance: 1,425 light years
• Spectral Types: A2 supergiant
• Position: 20h:41m:26s, +45°:16′:49″

Guide star reminder

Each month you’re encouraged to learn the new “guide” stars rising in the east about an hour after sunset. One reason for doing this is so you can then see how they move in the following months.

Deneb and the Northern Cross join several other guide stars and asterisms in the June sky. Again, if you have been reading these Posts for several months, be sure to find the stars, asterisms, and planets you found in earlier months. Early on a June evening these will include, from east to west, the following: Deneb, Vega, Arcturus, Spica, Saturn, Leo’s Rump (triangle),  the Sickle, Regulus, the Beehive, and in the northwest getting near the horizon, Pollux and Castor. You may also see Capella very near the horizon.

For more experienced observers looking to extend their knowledge of the skies this month, I highly recommend trying to track down two more asterism – the Northern Crown and the KeystoneOK, technically the Northern Crown (Corona Borealis) is a constellation. But I always apply the name to the handful of moderately bright stars that look like a half circle – a crown. As the chart below shows, these two asterisms are located on a line between Arcturus and Vega and they sort of divide that line into thirds. As with our guide stars and other asterisms, they will help you if you advance to finding other more interesting objects int he night sky with binoculars and telescope.

keystone-crown

For a printer-friendly version of this chart, click here.

The Crown itself can provide you with an interesting test of how dark your skies are since a couple hours after sunset on a June night it is well up in your eastern sky. It consists of a circlet of seven stars which can just fit within the field of view of wide-field binoculars – the example below shows an eight degree circle. It may be helpful to look at these stars with your binoculars, even if they don’t all fit in the same field of view at once. But to test how dark your skies are – and how transparent they are at the moment – wait until your vision is dark adapted, then see how many of these stars you can see. The numbers beside the stars are the magnitudes in decimals as given by Starry Nights software. However, I’ve followed the convention of not using a decimal point, since it might be mistaken for another faint star. So “41” means magnitude 4.1, for example. If you are seeing all seven stars you can be happy with your skies and these light-polluted times. In a truly dark location, however, this will be easy – but sadly such locations are rare these nights.

Read text above for explanation of how to use. Thenc lick on image to give you a larger view and luse the link below to download a printer friendly version. (Made from Starry Nights screen shot.)

For a printer-friendly version of this chart, click heret.

Look North in June 2012! We have a special ‘North Sky Triangle’ this month!

Click to enlarge. (Prepared from Starry Nights Pro screenshot.)

For a printer-friendly version of this chart, click here.

Yep, that’s Deneb, the guide star that is the subject of our “Look East” Post for June, gracing our “Look North” chart as well. In fact, besides Polaris we have three key guide stars in our northern sky every June, each of which is noted for, among other things, just how far north it is.

Of the three, Capella may be the hardest to find, for it is very near the horizon in the northwest around sunset. But if you have a clear horizon in that direction, you should still pick it up, especially at the start of the month. More prominent, however, are Deneb and Vega. These stars play the key role in one of the best known sky triangles – the Summer Triangle, but that triangle becomes easier to see next month and it will be in the eastern sky. For June it is fun to link Deneb and Vega with Polaris in what we’ll call the North Sky Triangle – and the linkage has some special meaning.

We just happen to be lucky to be living in an era when we have a bright star near the North Celestial Pole – Polaris. There’s no such bright star near the South Celestial Pole, and in other eras there is none near the North Pole either. But in the distant past – and in the distant future – Deneb actually will be the bright star nearest the pole – not as near as Polaris, but still a good general guide to it.

gyroscope precessionThat’s because the Earth wobbles as it spins on its axis in much the same way as a spinning top does. So the axis of the Earth doesn’t always point to the same place. It slowly makes a great circle around the northern sky, taking roughly 25,000 year to complete. Right now our axis is pointing to within a degree of Polaris. Not precise, but good enough so it is a ready indicator of true north. A mere 18,000 years ago Deneb was within 7 degrees of the pole and will be again around the year 9800!

This wobbling of the pole is really kind of mind boggling. I look at Polaris now, and it’s a bit short of 42 degrees above my northern horizon. But in a mere 14,000 years, Polaris will be almost straight over my head, and guess what will be the pole star then? Not Deneb, but its brighter – in our skies – companion, Vega. Of course none of us is likely to witness that event, but it’s still food for thought and gives us a sense of the majestic rhythms and time frame of the heavens.

And speaking of that time frame, as I write this it occurs to me that 14,000 years really feels like a very long time from now – while the 10-million-year age of Deneb doesn’t seem that long – and it isn’t, astronomically speaking. I’m not talking now about what your mind tells you about those numbers. I’m talking about your emotional reaction to them. I wonder if it’s similar to mine? This isn’t idle speculation. It’s central to our appreciating what we are seeing. But I think 14,000 sounds like a long time because it’s a number that fits into our day-to-day experience. Huge, but we can easily imagine 1,000 of something.  So imagining 14,000 of something comes easily to us. But few of us have any experience with one million. It would take about 11 days to count one million seconds and no one in his right mind is about to try it. So when we speak of the 10-million-year age of Deneb, or the five-billion-year age of the Sun, the numbers lose their emotional impact because they don’t relate easily to our experience. But 14,000 – well, we know, in an emotional sense, just how long that is!

Venus transit June 5/6, 2012 – you don’t want to miss this one!

UPDATE – The transit went well for many folks throughout the world. For a personal observing report and a few pics I took, go here. For lots and lots of pictures , go here.  And for other transit observing experiences, go here.

 

Transit as seen from Westport, MA through a hole in the clouds.

Study this NASA map to see whether you are slated to see all of the transit of Venus on June 5/6, 2012, or part near the time of local sunrise, or part near local sunset. (Click image for larger version.)

On June 5/6, 2012, most of the world will have the chance to see all – or part – of a once-in-a-lifetime  event – a transit of Venus across the face of the Sun.  CAUTION: To view this, even with the naked eye, you must use proper protective filters. Binoculars and telescopes must be equipped with such filters and if not, used only to safely project an image of the event – not looked through.

That said, here are three shots simulating the event as seen from Westport, MA. From this East Coast location we will see only the first couple of hours of the transit, then our view will be interrupted by sunset. Notice that Venus will appear to enter near the “top” of the Sun, This location and path vary with your position on Earth. (At the end of this post are several links. The second of these links gives you specific information on the time and the path of Venus across the Sun as seen from your location. In the images below, the Sun is festooned with sunspots and other features. Such features may or may not be seen depending on what is happening on the Sun at the time of the transit and on the type of solar filter used to view the event.

Predicted path of Venus transit across the face of the sun as seen in astronomical telescope (flips image horizontally) from Westport, Ma. Click image for larger view. (Prepared from Starry Nights Pro screen shots.)

Did you find the images exciting? Probably not. But it should give you some idea of what to look for on June 5, and there is no substitute for seeing the real thing as it happens.  There’s also no substitute for understanding what it is you’re viewing and why – besides the fact that there won’t be another such transit for more than a century. No wonder so many people are very excited about seeing it. I’ve already seen one such transit – as have many others – for these events come in pairs fairly close together, and the last one was visible just eight years ago.  But I still will make every effort to see this one, and if the weather forecast says my local view is likely to be obscured by clouds, I’m ready to drive a couple of hundred miles to get to some place that’s clear.

Here, in a nutshell, is why I find this event so exciting:

  • There won’t be another chance to see a transit of  Venus until 2117.
  • On display will be the full majesty  – and magic – of our gravitationally-powered solar system where Venus – a body almost as large as the Earth – passes directly between us and the Sun at a distance of bout 30 million miles.
  • More than 200 years ago many scientists risked life and limb to travel to distant locations on the Earth in attempts to view the transit and accurately time it.  Such observations, they hoped, would unlock the secrets of the size of our solar system – secrets that despite their best efforts remained hidden. As noted in the New York Times, “Sea travel was so risky in 1761 that observers took separate ships to the same destination to increase the chances some of them would make it alive.”
This event is covered in so many different ways with wonderful graphics, wonderfully accessible details about when you can see it from your location, and terrific stories, that I would be trying to reinvent the wheel to repeat it all here.  Instead, I urge you to take advantage of the links below.

Some useful transit links

Events May 2012: Ring of Fire in the West, the fattest Moon, thinnest Venus, and parade of twins

I would love to see the May “ring of fire” – an annular solar eclipse, but it’s too far away for me. However, if you live anywhere near the eclipse path which starts in Asia and ends in the western United States, May 21/20 could prove special. Sky and Telescope says that while the eclipse weather for Asia tends to be bad at this time of year, the weather tends to be very good in the Western United States. And I have to admit, one of the little fascinations of this event for me is it starts on May 21 and ends on May 20 – yep, time can run backwards ;-).

Of course, if you’re not in the eclipse path,  May offers some other choice viewing for the unaided eye and binoculars :

But first, a few more eclipse notes

Path for the May annular solar eclipse. Click for larger image and for many more detailed eclipse maps, see the links a couple paragraphs down.

OK, it’s not time that’s running backwards – it’s the shadow of the Moon across the Earth and the shadow starts in Asia on May 21, eventually crosses the International Date Line, and then ends in Texas on May 20.  And – just to be clear – an annular eclipse  is not the same as a total solar eclipse, nor as a partial eclipse.

The annular eclipse is better than the typical partial eclipse – which is still fascinating – but it is not the stunner that a total solar eclipse is. It is “annular” – the word means “ring shaped” –  because the Moon is so distant from the Earth at the time of the eclipse that it is not quite large enough to totally cover the Sun and so there will be a ring of light – thus “ring of fire” – at “totality” which is probably better thought of as “mid-eclipse” since it won’t be total.  The Moon will cover  94% of the Sun’s diameter, but that remain 6 percent will still generate a lot of light. It will be noticeably darker at any given location during those few minutes most of the Sun is covered, but it will not be nearly as dark as when there is  a total  eclipse.

For a full selection of detail eclipse maps and other information for different sections of the world, please go here. And for far more detail on everything to do with the eclipse, go here.

It’s a BIG – I mean REALLY BIG – full moon!

As noted, the annular solar eclipse occurs because the New Moon in May is so far from the Earth and thus appears so small that it’s disk does not cover the Sun. On May 5, when the Moon is full, it is closest to us in it’s orbit – as close as it gets at the time of full Moon in 2012 and thus gives us an especially large full Moon.

How large is large? Well, when it rises on May 20th on the East Coast  of the U.S. it will be right near it’s minimum distance of 221,457 miles and will show a disc of roughly maximum size – about 33′ 30″ in diameter.

And on May 20, when it is creating the  annular eclipse of the Sun, it will be very close to it’s maximum distance from us of 252,712 miles and it’s disc will be roughly 29’24” in diameter. (Of course it will be too close to the Sun for us to see that night, but in the next few days the crescent will emerge and that crescent will  include a lunar disc shining by the reflection of light from the Earth and  an especially small one at that.)

Why the “roughly” and “abouts” for sizes and distance in those sentences? Because the Moon is constantly in motion and constantly changing size and distance from us. So while there’s a correct size and distance for a specific instant – such as Moon rise at my exact longitude – we have to be more general when we’re using numbers that cover a date and time for Moon rise over different parts  of the Earth, or an extended event like the annular eclipse.

So will you be able to tell that it’s big? I mean, if you do the math you will  see  that we’re talking of a change from largest to smallest of roughly  four minutes of arc.  Can we detect such a change? Yes. Look at the images below. See a difference?

The moon when closest – and farthest – from us. To simulate the experience of two Moon’s at these different distances from us, click on the image, print the resulting picture, and tape it to the wall 12 feet away from you.  (Images from Fourmilab, by John Walker
– public domain)

Unfortunately you never get to see such a comparison live, in the sky. In fact, you will have to wait until  Nov 28, 2012  to see the smallest full Moon this year.  At that time it will be right out there near it’s maximum distance from us and show a minimum disk size. (Hmmm… would be fun to photograph the May 5 Moon rise near a certain landmark, then do a similar photo at the same spot  on November 28. nut. of course, it would have to be a portable landmark you move into place because the two will not rise in the same location – in fact, they will be quite far apart.)

Now, if you want to crunch the numbers, consider four minutes of arc – that is frequently quoted as the distance we can detect with our naked eye. So, for example, two stars that are four minutes of arc apart and the same brightness we could split without optical aid. So why is it obvious the Moon is bigger when it’s a difference of just four minutes?  Remember that  with the Moon we’re citing a diameter, but what we see is an area. The area turns out to be 16.75 squared time 3.14 = 881  vs  14.7 square C 3.14=679 – a factor of 202 – nearly one fourth!  So if you calculate the area of the Moon’s disk visible to us when nearest and when farthest away the difference is significant!

There’s one caution, though.  The Moon and Sun when near the horizon always – ALWAYS – look much larger than they do when high in the sky. This has nothing to do with their  actual distance from us, or size. Take a picture of that Moon near the horizon and the picture will show a Moon that looks much smaller than you remember seeing.  The reason is what’s commonly known as the Moon Illusion – and that a whole different story. For a formal discussion of a complex topic, take a look here.

Complex as the Moon Illusion is, when you begin to understand the constantly shifting position of the Moon – develop a gut feeling by watching the changes – you really begin to appreciate the incredible complexity of landing a space mission there. And those with long memories will recall that  landing on the Moon is hardly a slam dunk.

In 1959 they [the Soviet Union] launched Luna 1, which missed the Moon by 3,728 miles (5,998 km). They followed that flight with a spectacular circumlunar orbit by Luna 3, which gave us our first pictures of the far side of the Moon.

 The development of probes in the United States also revolved around the Moon at this time. After several unsuccessful attempts to reach the Moon with the Pioneer series, the National Aeronautics and Space Administration (NASA) launched the Ranger series. It planned to crash-land the spacecraft onto the Moon’s surface, taking photos up until impact. The first few probes were unsuccessful, but the last three– Ranger 7, Ranger 8, and Ranger 9–took over 17,000 pictures beginning in 1963.   source

So this whole business of the lunar orbit around us is complex and is really better thought of as the Moon’s orbit weaving inside and outside our own in the course of each month  as we both travel around the Sun.  So I hope the weather cooperates and you get to bask in May’s full Moon and contemplate our deceptively simple relationship to our companion planet. (Yeah – that’s another thing – many regard the Earth and Moon more as a double planet system – the moons of other planets are much smaller in relation to their planet than our Moon is in relation to us.)

And now that we have the Moon on stage, how about that svelte Venus?

Venus goes through phases like the Moon as well. But what’s interesting about the Venus phases is that it is “full” when it’s farthest from us – and it’s a thin crescent when it’s closest to us. That really changes the dynamic. With the Moon there’s no such relationship. It can be a crescent and close, or a crescent and far away.

That thin crescent in May 2012  grows to more than 56″ in diameter by the end of the month.  Yes, those are seconds or arc. It’s still much smaller than the Moon where we measure it’s angular size in minutes. Remember, one minute equals  60 seconds, so the full Moon near the beginning of May is about 35 times as large as Venus is to our eyes near the end of May.  Can we see something this small as a crescent? I think it would be very difficult with the naked eye, but handheld binoculars will magnify it  7-10 times – that makes its crescent form identifiable.

BUT IF YOU PUT THIS TO THE TEST, PLEASE BE CAREFUL. VENUS RAPIDLY APPROACHES THE SUN THIS MONTH. So I suggest if you try to see it in daylight, you do so in the early part of the month. It is a crescent on May 1, though at 44″ a smaller one, it is still large enough to be detectable. JUST AVOID LOOKING AT THE NEARBY SUN WITH YOUR NAKED EYE AND/OR BINOCULARS OR A TELESCOPE.  For more details on how to safely see Venus in Daylight  go here.  On May 1, 2012 Venus is still about 36 degrees from the Sun.  By May 10, 2012  it’s about 30 degrees away and by the 20th it’s 20 degrees away. That is really getting too close for comfort as far as I’m concerned. In the second half of the month I would only look for Venus after sunset – even when taking the precaution of putting a building between me and the Sun.  I value my eyes far too much to play games.

But the point is. we have some interesting dynamics at work here in terms of its brightness. You would think Venus would be brightest when it was “full” or near full – just like our Moon. But it isn’t. And you might think it would be brightest when it is closest to us – but then it’s just a thin crescent that we see, so it isn’t.  Actually, there’s a compromise position about one third of the way through May 2012 when it is both a crescent – less than 20% of the disc illuminated – and near it’s brightest at magnitude -4.7. After that it gets to be a larger crescent, but it also dims some because so little of the disk is lit. Still, even at the end of the month with just one percent illuminated it is shining at a dazzling  – 4.1.

But I  hasten to add that in that last week Venus will be more and more difficult to see. Fifteen minutes after Sunset it is just five degrees above the horizon ( at my mid-Northern latitude) and in the bright twilight would require a clear and unobstructed western horizon to see.

This plunge towards the Sun, is, of course, heading for that twice-in-a-century event, the Venus transit of the Sun.  We already had a shot in 2004 at a transit of Venus, but these events come in pairs. The June 5, 2012 transit is the second of the pair for this century.  I’m really looking forward to this one. For North America, only the first part of the transit will be visible with sunset interrupting it. Weather prospects are pretty problematic too.  I plan to set up special equipment, properly filtered for safely viewing the Sun, in my favorite location with an unobstructed western horizon. But I’m also scouting other locations within reasonable driving distance, if the weather looks more favorable  north or west of here. I’ll publish a separate post with more details for viewing the transit which will be widely available from different locations on Earth and provides a way to relive some wonderful scientific history. In the 18th and 19th centuries viewing a transit of Venus was regarded as the key that would open the door to being able to calculate the actual size of our solar system. That provided the impetus for some fascinating – and downright heroic – scientific expeditions around the world.

May’s Parade of Twins – Saturn/Spica, Mars/Regulus and the Real McCoy!

The “Heavenly Twins,” Castor and Polux are still with us in May, high in the West an hour or two after Sunset. But they are joined by two other closer pairs of bright “stars” that have a special fascination do to color contrast and motion. High in the southeast are  Saturn and Spica.  And high overhead and favoring the southwest are a third pair, good as such only for the start of the month, Mars and Regulus.

Click image for larger version of this chart – prepared from Starry Nights Pro screen shot.

The pairs present some really nice color contrast , something that will be more apparent if you look at them in binoculars. Saturn has a yellowish tint, while it’s companion, Spica, is an icy blue. Mars is orange-to-red, while Regulus is white with just a hint of blue. Castor is white, but Pollux has a yellow tint.  (For more on the color of stars, please go here.)

Quite a line-up, really, since all are very bright “stars.”  Castor, while bright, is the dimmest of the six.  It is magnitude 1.56 and the convention is to say that first magnitude  runs from magnitude .5 to magnitude 1.5.  So Castor  misses first magnitude  by hair where the others are either first magnitude or zero!

Pollux is magnitude 1.15.  Saturn starts the month at magnitude .5, then joins the zero magnitude class by climbing to magnitude .3  by the end of the month.  And Mars? Mars is the most fickle  of the group. It starts off the month a perfect 0 magnitude, but by the end has dimmed to .5, so it’s headed for the first magnitude class. It also breaks the twin pattern – that is, at the start of the month it is is less that 6 degrees from Regulus (magnitude 1.34), but it more than doubles that distance by the end of the month.  Saturn barely changes it’s relationship with Spica (magnitude .96), being about 4°50′  distance all month – and, of course, Castor and Pollux are, for all practical purposes, constant at a separation of 4°30′.

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