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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.

Events for April 2010: Venus, Mercury, the Moon, Mars and the Bees, Algol and some morning meteors

The “events” post assumes a new, chronological format this month. The idea is two-fold:

  1. Make it easy to mark your calendars for events worth making special plans to see.
  2. Give you a place to check before going out to observe to see if anything special is happening that night.

There is one category of special events worth checking on, however, that we don’t list here because they’re very specific to where you live and when you observe. These are events involving man-made objects in space – the passages of the International Space Station, Iridium flares, and other bright satellite and space craft passages. There are two excellent ONLINE sources for such events. I urge you to check both, see how they differ, and then make your own decision as to what works best for you.

  • The first is provided by Spaceweather, and you’ll find it by going to their Web site and clicking on the “Satellite Flybys” link on the right, or  by going directly here.
  • The second is the Heavens Above site, and while this requires you to register, the process is painless and free and the result is a lot of information that is specific to your location. You need to know your latitude and longitude, but you can get them by using the link in the “configuration” section near the top of the Heavens Above page. This is a one-time process. Once registered and logged in, study the menu – there’s a wealth of information on satellites and many other things.

April 2010 Astronomical events

Note: While many events are visible throughout the world, the exact time and location in the sky can be dependent upon your latitude and longitude. Since I’m in the mid-northern latitudes (41.5N, 71.1W), specifics, where place-dependent, are calculated for this location.

April 1-15 – Mercury makes what Sky and Telescope calls its “best evening display” of the year, and brilliant Venus will point the way to it. Mercury is brightest at the start of the month and fades significantly by the 15th. But at no time can it hold a candle to Venus, which will be easily found at magnitude -3.9. Just look about a fist or two above the horizon and a tad south of where the Sun set an hour before.

While Mercury is visible to the naked eye and was well-known to ancient observers, it is easiest to find if you use binoculars to pick it out of the twilight. Start half an hour after sunset and locate Venus. In the first few days Mercury will be in the same binocular field with its brighter companion. Here is what the scene should look like on April 1 – no fooling!

From Starry Nights Pro screen shot.

This close relationship holds throughout the 15-day period, though after the first week Mercury starts dropping off towards the north, and by the 13th it may no longer be in the same binocular field – depends on the binoculars used. The 15th is special, though, for Mercury then has a close encounter with a very young Moon. (Scroll down to the listing for the 15th for details.)

April 3 – A very bright, 19-day-old gibbous moon brushes by the bright, red guidepost star Antares at dawn. For me the best time to see this event will be, I believe, between 5:15 and 5:30 am EDT. I’ll be watching in binoculars. The moon will be getting closer and closer, but dawn will be breaking and starting to wash things out. If you live farther west, you’ll see an even closer encounter. Here the separation will be almost a full degree. That is, when viewed with dark skies they will still appear separated by a space greater than the width of the moon itself. The closest approach – about an hour after sunrise for this location – will still leave a gap just about the apparent diameter of the moon itself. I like these close encounters as a reminder of the motion of the moon, which goes counter to the spin of the Earth.

April 14 – The most convenient minima of Algol, the “Demon Star” occurs this month on April 14th at 9:40 pm. Should be fun to check on it about an hour after sunset, then again around 9:40 pm. The only problem – it’s getting very low in the northwest. At 9:40 it will be just 14 degrees above the horizon and will set in a couple of hours – hardly ideal conditions, so I expect this will be the last time I’ll check on it until next fall. For more details on Algol and its eclipses, see the post here.

April 15 – The Venus and Mercury show goes out with a bang, as a 1.4-day-old crescent moon joins them about one fist above the horizon 30 minutes after sunset to the west northwest. Again, Venus should be easy to pick out with the naked eye. Once you have it, use your binoculars and get it in view with them, then move down and to the right just a bit and you should encounter Mercury and the crescent moon. The exact age of the moon and closeness of this encounter will depend on your longitude. From where I sit (41.5 degrees N latitude and 71.1W longitude), the Moon will be just 1.4 days old and less than a degree away from Mercury, which will have dimmed to magnitude 1.5. I don’t expect this to be easy to see. But it all can be visible to the naked eye given an unobstructed western horizon and very clear skies. Once you’ve located it with binoculars, you probably will be able to see it with the naked eye. All depends on a modicum of skill and a lot of luck 😉

Here’s what it should look like.

From Starry Nights Pro screen shot.

April 16-18Mars and the Beehive (M44) mix it up in a delightful pairing for binocular users. Mars has begun its eastward movement against the background stars, and in doing so rolls right between the Beehive cluster and Asellus Borealis, the “Northern Ass.” Actually, this will make a nice binocular grouping from about April 11 to the 23rd, so if the weather doesn’t cooperate on the best nights, try one of the alternatives. I love it when planets get near distinctive stars and star groups because it’s easier then to chart their motions from night to night. So I would urge you to look at Mars with your binoculars at least two nights during this time frame and use the chart below to mark its position. (For other charts to help you locate Mars and the Beehive, see this posting. )

Click image for larger version. Prepared from Starry Nights Pro screen shot.

Download a printer-friendly version of this chart here.

April 22-23 – Greet the dawn with some meteors! Sky and Telescope says these are are the best dates for the Lyrid meteor shower this year. But with the moon offering interference this is a pre-dawn event and the Lyrids seldom put on a big show. If it’s clear, my plans are to be out about 2:30 am. By then the just-past first quarter moon will have set, so it won’t offer interference, and I’ll have a couple of hours before the pre-dawn light begins to wash out the eastern sky. The next night, the Moon will be a bit brighter and sets about 45 minutes later, so there won’t be as much dark-sky observing time available. I like to be aware of this kind of event, but I don’t plan my observing around it. I go out to observe other things and pause once in a while to sit back, look up at the sky for several minutes, and hope I get lucky. Hey – if it’s clear and I can do that, I’m already lucky! 😉

April 24, 25 – Venus and the Pleiades have a relatively close encounter making a nice binoculars view. Actually, they should both fit in the same low-power binocular field from about April 20 to the 28th, but it’s on the 24 and 25 that Venus is closest as it scoots past. The Pleiades are getting closer to the western horizon with each night. Venus is moving in the opposite direction, higher in the sky each night. (It will be our western evening star right into September.) By the 28th it’s about halfway between the Pleiades and the Hyades, though the clusters are getting so low they’re losing much of their luster. Getting back to the 24th and 25th, Venus will be simple to spot. Look for it about an hour after sunset and about one fist above the horizon to the west northwest. Then look for the Pleiades to the northwest of Venus in the same binocular field. Sweeping a little to the southeast of Venus (up and to the left) with your binoculars you should pick up the Hyades and Aldebaran, but not in the same binocular field as Venus.

Look North in April! See Mizar – the best thing since – well, since sliced bread!

In April the Big Dipper is climbing high overhead in the northeast and starting to pour its contents into the Little Dipper – not a very good idea, but fun to contemplate. Meanwhile, the only double star pair where both stars have proper names – Mizar and Alcor – is high in the northeast and ready to challenge your eyesight and boggle your mind.

Mizar is the middle of the three stars that form the handle of the Big Dipper – the same three that we use as an arc to trace a path to Arcturus. (That reference is explained in this month’s “Look East” post.) Wait until an hour or more after sunset, then focus on that center star. Is it one star – or two? For my old eyes it is one. And since my eyes are not that bad, I question those who say this is an “easy” test of eyesight. But lots of people do indeed see two stars there when they look carefully. Maybe you’re one of them. If you’re not sure, or can see just one, take a look with your binoculars. Now you certainly should see two.

The brighter of the two is Mizar, the fainter one Alcor. More on that in a minute. First, here’s our northern sky for this month.

Arrows indicate directions in the sky where north is always the direction towards the north celestial pole, and west is always the direction the stars appear to move. Click image for larger view. (Developed from Starry Nights Pro screen shot.)

Download a printer-friendly version of this chart here.

And here’s what you should see when you look with binoculars at the Big Dipper’s handle.

Zooming in on the center star in the Big Dipper's handle using binoculars, you should see it is really two stars - Mizar and Alcor. Click image for larger view. (Developed from Starry Nights Pro screen shot.)

The words “double star” simply mean that a star that appears as one to our naked eyes, is seen as two when optical aid is used. But they may simply be two stars that are closely aligned, yet in reality very far apart and have no real connection to one another. “Binary star” is the term used for two stars that are gravitationally linked to one another. So here’s the double rub with Mizar:

  • When you are looking at Mizar and Alcor, you probably are looking at six stars, not two!
  • Scientists still dispute whether Mizar and Alcor are a true double, even though they have been discovering this system since 1650.

My “sliced bread” reference figures into this dispute in a roundabout way. I have trouble remembering things. So when I wanted to remember the approximate distance to Mizar – 80 light years – I asked myself what interesting thing was going on 80 years ago that can help me remember the distance to these stars? And the answer – given a little research – was that about 80 years ago America was introduced to sliced bread all packaged neatly. Actually, sliced bread was first introduced in 1928, according to Wikipedia, but it was in 1930 that the first national marketing campaign began for “Wonder Bread.” Wonderful. But don’t let the different dates bother you because the exact distance to these stars is in dispute.

And Mizar alone is a lot more interesting than sliced bread.

Even a small telescope reveals that Mizar itself is a beautiful double! That’s what was revealed when a telescope was turned on it in 1650. But no telescope can reveal to the eye that these two stars are in fact, each a double! The stars in each pair are so close to one another that only an instrument known as an interferometer can reveal them. So what we see as Mizar is in fact four stars.

But what about Alcor? The Hipparchos satellite, the best modern source for star distances, found Mizar to be 78.1 light years away and Alcor to be 81.1. Those are great ball park figures and good enough for the sliced bread reference. But they may be wrong. The astronomer James Kaler wrote a few years ago in his book “The Hundred Greatest Stars” that these distances may be wrong – in fact, some evidence suggested then that Mizar was actually farther away than Alcor. Kaler concluded in his book that they are “probably paired.”

But now comes more evidence as reported in the current (2010) Wikipedia reference to Mizar:

. . . in 2009, it was reported by astronomer Eric Mamajek and collaborators that Alcor actually is itself a binary, consisting of Alcor A and Alcor B, and that this binary system is most likely gravitationally bound to Mizar, bringing the full count of stars in this complex system to six.

So what our naked eye reveals as one or two stars, may indeed be a complex system of six stars! Which in my mind says that slicing up Mizar and Alcor this way may be – well, may be the best thing since sliced bread and just the sort of thing that makes observing the stars such a treat for the mind!

Look East: April brings the World’s Fair Star and Saturn!

I Click image for larger view.

Arcturus isn’t universally known as the “World’s Fair Star,” but it should be. Its light bridged two World’s Fairs , making a physical link between the one in 1893 and a second 1933 – both held in Chicago. It’s intriguing that the general public was excited enough about science – in the middle of the Great Depression – to make such a link attractive to the Fair’s promoters. Light from Arcturus – believed at that time to be 40 light years away – was captured by the 40-inch refractor at Yerkes Observatory and used to turn on the lights for the 1933 Fair.

This put the public spotlight not only on Arcturus, but it raised consciousness about the vast distance between us and that star, since the light being used had started its journey during the 1893 Fair and arrived just in time to start the next Fair. When you know light can circle the Earth more than seven times in a single second, you start to understand just what an incredible journey that was.

Of course Arcturus has many other distinctions. For one thing, it makes a perfect connection with the best known asterism in the sky, the Big Dipper. To find it, all you have to remember is “follow the arc to Arcturus.” That should keep you from confusing it with the brightest “star” in our eastern sky shortly after sunset on these April nights – the ringed planet Saturn. Saturn will be higher and to the south of Arcturus. Even binoculars won’t reveal the rings, however – for that you need a small telescope. Here’s the chart.

The name "Arcturus" derives from Ancient Greek and means "Guardian of the Bear." It is the brightest star in the constellation Boötes. Click image for much larger version. (Chart derived from Starry Nights screen shot.)

Click here to download a printer-friendly version of this chart.

Another way to remember where to find Arcturus is its name, derived from ancient Greek, which can be translated as “Bear Watcher.” That’s because Arcturus looks like it’s keeping an eye on the “Great Bear,” Ursa Major, as both circle the northern pole.

You can also think of the magnitude system by which we rate the brightness of stars as starting near Arcturus. At magnitude -.04 it’s very close to zero. Its absolute magnitude is also pretty close to zero since absolute magnitude is defined as how bright a star would be if it were about 33 light years from us, and Arcturus is actually about 37.6 light years from us. That makes its absolute magnitude -.29.

Arcturus has the distinction of being the brightest star in the northern celestial hemisphere, but this is splitting hairs in several ways. It means it’s the brightest star north of the celestial equator. Sirius, now over in the southwest, is obviously brighter. But Sirius is south of the celestial equator. Both stars are located close enough to the celestial equator so they can be seen from most places on Earth.

But Arcturus (-.04) also wins this “brightest star in the northern hemisphere” distinction by another hair. Remember that the lower the magnitude number, the brighter the star. Both Vega (.03) and Capella (.08) are north of the celestial equator, and the difference in brightness between Arcturus (-.04), Vega (.03), and Capella (.08) is only a tad more than a tenth of a magnitude. For practical purposes, they all look the same. But in practical terms, making the comparison by naked eye is – well – very impractical. Capella is currently fairly high in the northwest. But next month, when Vega is high enough in the east to see well, Capella will be rather low in the northwest. At that time Arcturus should look brighter – but its actual brightness will be aided by the fact that it is high over head at that time, so you are seeing it while looking through a lot less air than you will be when looking at Vega or Capella. Besides, visually trying to compare stars that are this far apart is next to impossible. I simply think of all three as magnitude zero and leave the hair splitting to the scientists and their instruments.

Oops – we interrupt this program for a bulletin from 1907!

Yes, having just written how impractical the naked eye comparison is, I found this passage in “The Friendly Stars” by Martha Evans Martin, a book that was published more than a century ago:

Arcturus and Capella are so nearly equal in brightness that astronomers differ as to which outranks the other, even when they measure their light with a supposedly accurate instrument and a trained eye. To my own eye Arcturus outshines Capella, and on asking various inexperienced persons for off-hand opinions as to the relative brightness of the two stars, I have invariably had an answer in favor of Arcturus. The best authorities, however, make Capella a shade brighter.

Oh my! And now with 100 years of scientific progress, the verdict is that Martha Evans Martin and her causal observer friends were correct – and the “best authorities” wrong. Arcturus is the brightest. So much for my idea that you can’t tell the difference with the naked eye! Give it a try and see what you think. (You can find a chart for Capella and more details about that star in this post.)

Since we’re ranking stars, however, Arcturus is actually fourth on the list of brightest stars – two others that are ahead of it, Canopus and Rigel Kentaurus, are not seen by observers in mid-northern latitudes.

While Arcturus radiates a lot of energy, much of it is not in the form of visible light. It has what’s known as a “peculiar spectrum” and radiates much of its energy in the infrared portion of the spectrum. This means that to our eyes it doesn’t look as bright as it really is.

One more deception of sorts: This brightness is not because Arcturus is so big – well , yes it is, but not big in terms of the amount of stuff in it, but big in terms of surface area. If you’re measuring the amount of stuff that makes up Arcturus – its mass – it is about the same size as our Sun. But Arcturus has a much greater surface area, so think of it as a hugely bloated version of our Sun. (Keep that in mind when you look at the comparison sketch on the left.) It is a much older star and is now going through its red giant phase – something our Sun will probably do several billion years from now, burning the Earth to a cinder in the process.

Vital stats for Arcturus, also know as Alpha Bootes:

• Brilliance: Magnitude -.04 , brightest star in the celestial northern hemisphere; shines with the luminosity of 215 Suns.
• Distance: 37 light years
• Spectral Type: K1 Giant
• Position: 14h:15m:38s, +19°:10′:5

Guideposts reminder

Each month you’re encouraged to learn the new “guidepost” stars and asterisms 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. So if you have been following – even if this is just your second month – look for the previous guidepost stars and asterisms that you have learned and that are still with us in April. Here’s the list from east to west.

Arcturus, Leo’s Rump (triangle), The Sickle, Regulus, the Beehive, Mars, Procyon, Sirius, Pollux, Castor, Betelegeuse, Orion’s Belt, Rigel, Capella, the Kite, Aldebaran, the Winter Hexagon, the Pleiades. Venus is our bright “evening star” in the west, followed by Mercury which, as usual, plays hard to get. (For when, where ,and how to find Mercury, see the “Events” posting for April. For our northern guideposts, see “Look North” for April.

Events: March 2010 – Venus takes center stage, west and a marathon night

The date I’m circling on my calendar for this March is the 16th – that’s when I want to go to my favorite spot that has a clear western horizon and catch the sun setting practically due west, Venus emerging in the early twilight, and with luck, the thinnest of crescent moons next to it. Then I’ll wait another hour and see if I can detect the ghostly glow of the zodiacal light, and for the rest of the night – with telescope handy – I have my choice of the entire catalog of Messier objects, something that can only happen in mid-March!

Of course, the only thing magical about March 16 for me is the combination of Venus and the crescent moon. Truth is, this combination will be available in an easier to detect form the next night as well – and Venus is available in the western sky all month! But on March 1, half an hour after sunset, it is just 4 degrees above the western horizon. By March 15 it is nearly twice as high, half an hour after sunset, making it much easier to see. By the end of the month it is more than 11 degrees above the horizon at that time, and if you can find it in your binoculars, you’ll see another bright “star” right below it in the same binocular field – that will be fleeting Mercury. Mercury first puts in an appearance about the 20th of the month, but doesn’t really become easy to see until the last days of March.

This chart is good for mid-northern latitudes. The exact position of Venus and the Moon will change depending on your location on Earth. Do not look until after sunset and be especially careful when using binoculars to wait until the Sun is well below the horizon. Click image for larger view. (Chart prepared from Starry Nights screen shot.)

If you want to see both Venus and the crescent moon on March 16, first notice exactly where the Sun sets. Wait 10 minutes, then start scanning that area of the horizon for a bright “star.” Venus will be easier to find than the crescent moon, though on the next day the moon will be higher and easier to find. If you find Venus, the Moon will be to the right (northward) and lower. It may just barely fit in the same binocular field, but most likely you’ll have to let Venus slip out at the upper left and be looking for the moon to come into view in the lower right.

The zodiacal light is available any time of year, but is much easier to detect in the evening sky about 80 minutes after sunset in February, March, and April – with March being the best. For more details about what it is and how to observe it, see this posting.

As to seeing every one of the objects in the Messier catalog, you should first understand that only a few of these are visible to the naked eye, the most famous being M45, the Pleiades. The Astronomical League classifies 42 of the 110 Messier objects as “easy” to find with ordinary binoculars – assuming you know just where to look. For the rest you really need a small telescope, or large astronomical binoculars. During most months at least some of them are obscured because the Sun is in the same section of sky as they are. They can also be drowned out by the light of the moon. But in mid-March, each year, nearly all Messier objects are far enough away from the Sun to be seen, although two of the 110 (M74 and M30) are still so close to the Sun that they are very difficult targets.

Since the mid-1980’s this idea of prime time for Messier objects has led to an extraordinary “sport” of sorts where amateur astronomers pick a night in mid-March, and stay up all night with the avowed goal of finding each of the 110 Messier objects. They call it a Messier Marathon. Frankly, I have never tried it and the idea doesn’t have much appeal to me because when I find one of these fascinating objects I like to stay with for at least half an hour. So the most I would see in a 12-hour night is a measly 24! Still, I like the idea of staying up all night and looking at different Messier objects as our spinning Earth brings each into view.

This year the key date for the Messier Marathon is March 13. That’s a Saturday night, and there will be no interference from the Moon. So if this is your thing, hope for good weather. As with all astronomical events, even when they happen every year, it is still rare when the weather and your personal schedule cooperate so you can enjoy them fully. Of course, if work is no issue you don’t have to be strict about the date. Any time within a week either way gives you a good shot at finding nearly all of the Messier objects in a single night.

Oh – and don’t forget the equinox on March 20th.

I love the equinoxes for their long nights, moderate weather, and sense of cosmic balance. This is the time when the Sun is half way in its journey north – or in September, in its journey south – and is rising and setting very close to due east or west. This year the vernal equinox occurs on the afternoon of March 20th in my Eastern Time Zone and that means sunset on that day is pretty much due west. OK, technically it’s a bit off target, but astronomers are used to “close enough being good enough.” Even Polaris, the “pole star,” is nearly a degree off the north celestial pole, but most of the time we treat it as if it’s right on. If you’re new to observing the night sky, this is a great time of year to get your bearings at your favorite observing location. Watch either the sun rise or the sun set and note the landmarks near it – that way you’ll have a quick way to identify the eastern or western point on your horizon. (You could use a GPS instead, but I’m an incurable romantic and would rather use the motion of Earth and Sun. )

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