Session 1B: The March Sky
There are two goals here. First, to help you know what you can find in the night sky during the next few weeks. Second, to help to find your way around the night sky so you know how to find those things.
Its All Angles
When astronomers measure positions on the night sky, they use angles. There is actually a well-defined coordinate system used, but we won’t be covering that here. To find things, all you need is a starting point and a way to measure angles. If you’re a draftsman, an architect, a surveyor, or you play billiards, you probably have a good, intuitive sense of angles. The rest of us need some help.
You’re probably used to thinking of angles as those itty-bitty marks on a protractor. While they are, they are not the distances between those marks. On your typical 6-inch diameter protractor, 1° is a bit more than 1 mm (1.33 mm) at the edge of the protractor. But if you had a 1-meter diameter protractor, 1° would be nearly 1cm (8.7 mm, about 1/3-inch). The “itty-bitty” part of the impression starts to fade.
At their heart, angles are ratios: the ratio of a size to a distance. So that 1.3 mm length at 6-inches is the same as the 8.7 mm at 1-meter. And it’s the same as 2-full moon diameters (7000 km) at the distance of the moon (385,000 km).
Okay, so what, you’re thinking…do I have to hold a protractor up to my eye to find things? No….
The Human Protractor
Assuming you have at least one good eye and one arm and hand, you already have everything you need to approximate positions on the sky.
- Stretch out your arm and spread your fingers open wide: the angle covered from the tip of your pinky to the tip of your thumb is about 20°.
- Now make a fist, again with your arm stretched out: the angle covered by your first (keep your thumb tucked under) is about 10°.
- Hold up three fingers, your pinky, ring finger, and middle finger: they cover about 5°.
- Keep that arm up! Stick up your thumb: it covers between 2° and 3°.
- One more…stick up your pinky finger: it covers about 1° to 1.5°, between 2- and 3-times the apparent size of the moon.
Try this: the next time there is a full moon, hold up you pinky at arm’s length. Which is bigger? That is, can you cover the moon with your little finger (try with only one eye open)? What about near moonrise (evening) or moonset (morning) when the moon is near the horizon and seems so big; can you cover it then?
North to Alaska…Uhm, Manhattan
Yes, from here in Bay Ridge, Brooklyn, north is more-or-less toward Manhattan. On the one hand, it’s easy to find north by looking for that glow from the city. Or maybe not if you’re having trouble figuring out which part of the glow is from Manhattan… It’s pretty hard to see much in the sky, even in directions other than the north. But, with north approximately determined, where are the fun things?
In the South
At this time of the year, March, Orion is high in the southwest as soon as it gets dark. Orion is full of fairly bright stars so it is easily seen even from here in Brooklyn. The three stars of his belt are very distinctive and nearly every culture has some story related to these stars. If you’ve been able to find Orion in the night sky, you can probably pick him out of the star chart below.1
[img_assist|nid=746|title=Star Chart: Orion, March 20 8:30 pm EDT|desc=|link=node|align=middle|width=462|height=640]
The above image is a printable version (linked to bigger versions). Below is the same view with guides.
[img_assist|nid=747|title=Star Chart: Orion, March 20 8:30 pm EDT, with guides|desc=|link=node|align=middle|width=462|height=640]
The first thing to note about Orion is his size. The angle from the star in the lower right (Rigel) to the one in the upper right (Betelguese) is over 20°! When you look at constellation pictures its easy to get the impression these are small patterns in the sky, but most constellations are really cover a big chunk of the sky. It’s more than another 20° to reach the top of Orion’s upraised club. Here in NYC, you may not be able to find the stars in his club unless you can get well away from any local street lights.
Orion has several “famous” stars among them Betelgeuse in his upper left shoulder. Betelgeuse means “armpit of the giant.” Betelgeuse is a red giant star. It’s diameter is larger than the diameter of the orbit of Mars around the Sun! For almost all stars, even the Hubble telescope sees them just as points of light. But Betelgeuse is one fo the very few stars which can be imaged as a small disk.
Opposite Betelgeuse is Rigel “the foot.” Rigel is much smaller than Betelgeuse and it would “only” swallow Mercury and Venus.
Just to the west of Orion is Taurus. It’s easy to imagine Orion battling the bull from this arrangement, but some of the oldest drawing we have show Orion facing east, not looking at Taurus at all! The stars in Taurus aren’t supposed to form a complete picture of a bull, just his head and shoulders. I can’t really picture most of bull, but the large V-shape does make one think of the bull’s horns. The brightest star in Taurus is Aldebaran which may appear to your eyes with an orangish tint.
Just to the east and a bit south (closer to the horizon) is Canis Major, the Big Dog, one of Orion’s hunting dogs. This dog is marked by the brightest star in the night sky during this time of year, Sirius. The name “Sirius” comes from a Greek word which means “scorching.” Sirius is bright for two reasons: first is is very close in astronomical terms, only 9 light years away (53,000,000,000,000 miles, more or less); second, it is about twice the diameter of the Sun which makes it both hotter and intrinsically brighter. Sirius is about 10,000C while the Sun is “only” 6,000C. Just to the east an south of Sirius is the dog’s front leg. The star her is called Miram and takes its name from the Arabic word for “announcer;” it announces the imminent arrival of Sirius.
One a very clear night in Brooklyn, I can see most of the stars shown in the image above to make out this constellation. Even on hazy nights, if you can see any stars at all, Sirius will likely be among them.
The constellation Monoceros, the Unicorn, is also drawn, but I’ve never really been able to pick out the stars of this pattern from Brooklyn. There are a scattering of stars in that area that manage to poke through the city sky glow, but most are lost.
Orion’s other hunting dog is Canis Minor, the Little Dog, and it’s brightest star is Procyon. You’ll notice on the chart the whole stick figure consists of only a single stick! Procyon is not nearly as bright as Sirius, but it is still quite bright an easy to find from here in Brooklyn.
While Orion gets all the glory with its large number of bright stars and easily recognized pattern, another constellation appears above his head: Gemini, the Twins.
Gemini is really close to being overhead. In the charts, you can see Orion down below Gemini which should help to you get oriented.
The center of the circle for the guides represents the zenith, the point directly overhead. The two bright stars near the “top” of Gemini are Pollux (on the left and slightly lower) and Castor (to the right and slightly higher). If it’s really clear, you can probably trace the outline shown in the images and see most of the stars in this chart. If it’s a bit bright either due to sky glow (which is aggravated by high humidity) or if the moon is full and up, you may have trouble seeing anything more than Castor and Pollux and the star in the left “foot” of the constellation. Ah, but what’s that bright blob just to the right of the stick figure? That’s Mars! Mars is currently receding from us as we speed away in our orbit. Even the views in a telescope are not very good, but you can still see it with your naked eye.
In the East
Rising in the east around the same time is Leo, the lion. On March 20, the full moon will wash out most of the fainter stars in Leo and even make the brighter ones hard to see. However, two stars will be quite easy to spot. One is Regulus, at the base of the “backward question mark” that forms the head and shoulders of Leo. The other, slightly below and to to the left (east) of Regulus is not a star; it’s the planet Saturn!
[img_assist|nid=748|title=Star Chart: Leo, March 20 8:30 pm EDT|desc=|link=node|align=middle|width=462|height=640]
Below is the same image with guides.
[img_assist|nid=749|title=Star Chart: Leo, March 20 8:30 pm EDT with guides|desc=|link=node|align=middle|width=462|height=640]
This pattern of stars been associated with a lion for a long time, since at least 2200 BC. The brightest star that we see is Regulus. Regulus comes from Latin; we get the word “regal” from the same root and this constellation has also been associated with “kingship” for a long time.
Regulus is about 6 times the diameter of our Sun. But there’s another star in Leo which is even bigger. It doesn’t look so bright to us because it is about 25 times further away. Eta Leonis (which doesn’t even get a proper name!) is so much bigger and brighter that if we lived on a planet orbiting Regulus, Eta Leonis would be the brightest star in our night sky. If it were as close to us as Sirius (9 light years), it would 50 times brighter that the planet Venus appears to us and it would actually be visible in the daytime!
Regulus is also a double star but you need a telescope to see its companion. The companion star orbits Regulus at a distance about 100 times greater than the distance of Pluto from our Sun.
Another double star is Algieba (“lion’s mane”) but again you need a small telescope to see both stars. Each year, in November, the Leonid meteor shower occurs. It’s called the “Leonid” shower because the meteors appear to be coming from the constellation Leo. All meteor showers get named for the constellation from which they appear to come. For the Leonids, the radiant, or central point from which all the meteors appear to be coming, is just north of Algieba.
There are also a number of galaxies which can be seen in Leo. However, we’re not going to talk about those today because they are too faint to be seen without a telescope and without leaving the city for darker skies.
The last star I’ll mention by name is Denebola. “Deneb” means “tail” and there are several constellations which have stars whose names have the word “deneb” in them. The most famous can’t be seen until later in the year; that’s Deneb, the tail of Cygnus the swan.
1 All of the starchart images were created using Starry Night Pro 6 on Windows XP and manipulated with The GIMP on Linux.