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  • Rapt in Awe

    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.

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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