The Winter Hexagon

Good afternoon everyone, and welcome to the late-January edition of this “skywatching for teachers” newsletter!

As I mentioned in the last installment, there isn't anything really breathtaking happening in the sky this month. February and March will probably be similar, but then we'll have some more dramatic events later in the spring, including a “supermoon eclipse” in May. However, these January evenings are wonderful for simple stargazing. One of the largest and brightest assortments of stars is now beginning to decorate the evening skies. On a “world map of stars”, this area could be thought of as one of the two major “continents” of the globe, and it is rising earlier and earlier and climbing higher and higher every evening. Furthermore, if you can go stargazing in a dark location, you can see the lovely Milky Way arching high overhead these evenings, stretching majestically from the northwestern horizon, across the top of the sky (i.e. through the “zenith”), and back down to the southeastern horizon. So try to find a dark place to do some basic stargazing in the next week or three, and just enjoy the beauty of the night sky.

Let me start with a brief discussion of the planets, and then I'll provide a quick tour through a map of this stellar “continent” in the night sky.

The often elusive Mercury likes to play hide-and-seek near the sun, and this coming week will provide another good opportunity to hunt for it. There won't be any particularly helpful landmarks near it, but you probably won't need them. Mercury will be near its peak, and much brighter than usual, over the sunset. Look for it over the western horizon 40-50 minutes after sunset. The “greatest elongation” of Mercury will be tomorrow the 23rd of January, and after that it will start to approach the sun again. If you've been following the progression of Jupiter and Saturn into the sunset, you have lost Saturn by now, and probably Jupiter as well. They have finally disappeared behind the sun. They will reappear over the sunrise in February. (A short time ago, the Jupiter-Saturn-Mercury trio could be seen in the evenings over the sunset. By the end of February, Mercury will have completely reversed its position, at which time the same trio will then be visible in the mornings over the sunrise.) Mars is still high in the south in the evenings, but much dimmer than it was several months ago, when it was at opposition.

The Winter Hexagon

Orion is probably even more well-known than the Big Dipper. It isn't up as much as the Big Dipper, but it contains some of the brightest stars in the sky, close together in an easy-to-recognize shape. Around Orion are several other bright constellations, including the brightest star in all the sky (if you don't count the sun). Some parts of the night sky are dark and hard to recognize, but this wall of the night sky is one of the brightest and easiest to recognize. It is visible much of year, but it is especially prominent for northern hemisphere residents in the southern sky in winter evenings. For a map, click the following link. (I designed this as a worksheet for classroom use, so there is a blank version to color and label, and a filled-in answer key.)

We might as well start our tour of this bright constellation concentration with the most famous constellation within it: Orion. Find three bright stars in a row, surrounded by an upright rectangle of even brighter stars. The three stars in the middle are “Orion's Belt”, and the constellation is Orion. (Orion is standing up straight in the worksheet, but if you go out early in the evening, you will probably find him tilted to the left, still rising in the southeastern sky.) The upper left corner of Orion is another star with a funny name. The name Betelgeuse derives from Arabic, and if you pronounce it properly, it should sound something like bet-el-jooz...but most English-speakers just call it “Beetlejuice”. Betelgeuse is one of the brightest stars in the sky. If you make a list, it's number 10. The star in the lower right corner of Orion is even brighter. It is number 7, and its name is Rigel. If you've never noticed that stars have colors before, try comparing Betelgeuse and Rigel. If you are a fan of Harry Potter, you may recognize the name of the star in Orion's upper right corner: Bellatrix.

Around and within Orion are many dimmer stars. If you can see them, you can try to make a head, a sword, and a shield. There's also a club raised over his head, but you need to see really dim stars for that. However, even in city skies, you might be able to make out the sword dangling from Orion's belt. If you have a pair of binoculars, wait until the sky is very dark, then point your binoculars at Orion's sword and see if you can see anything interesting.

Below and to the left of Orion is a really bright star—the brightest in the sky. This is Sirius (another name which Harry Potter fans will recognize). The star above and to the left of Sirius is Procyon. Do you notice something interesting about Betelgeuse, Sirius, and Procyon? These are three of the brightest stars, including the brightest star, and they form an almost perfect equilateral triangle in the sky. This is the Winter Triangle.

If you are familiar with the celestial sphere, it may be worth pointing out that you can use Orion as a helpful compass constellation. The “equator of stars” (or more precisely “celestial equator”) runs sideways across Orion, right through Orion's belt, and right across the Winter Triangle. This means that Orion's belt lies exactly halfway between the north pole of stars (“north celestial pole”), and the south pole of stars (“south celestial pole”), and it makes his belt a valuable signpost in the sky. Orion's belt is probably the best marker in the sky for finding the equator, and thus also due east or due west on the horizon. If you ever see Orion's belt touching the horizon, that means you are facing exactly east or west. (In the northern hemisphere, Orion will be tilted to the left when he is rising in the east, and he will be tilted to the right when he is setting in the west.) If you can extrapolate the equator in your imagination across the Winter Triangle and across the sky, you can find approximate east and west any time you can see Orion.

The constellations for Sirius and Procyon are traditionally seen as a pair of dogs—hunting companions for the hunter Orion. What are Orion and his two dogs hunting? How about a rabbit? The stars below Orion are traditionally known as Lepus, the rabbit. (For possible ways to draw Canis Major and Lepus, you can download my worksheet Orion and Friends, and I also recommend the book The Stars, by H.A. Rey, the author of the Curious George series.)

Note that Canis Major and Lepus are fairly low in the southern sky. I could find them without trouble when I lived in Southern California, at a latitude of about 33°. In Iowa, at a latitude of about 43°, they still easily clear the southern horizon. But at latitudes any higher than 50° or so, you may have difficulty finding Canis Major or Lepus. Depending on how much clutter and haze you have on your southern horizon, they might be partially hidden or lost in the haze.

Above Orion and to the right, we can find a “V”-shaped formation of five stars, with the star capping the lower side being much brighter than the others. The “V” shape might make you think of the horns of a bull. This constellation is named Taurus, the bull, and the bright orange star at the “eye” is Aldebaran. We can even extend the horns if we want, to make a long-horn bull, using two stars above Orion. If you look closely at the stars of the “V”, you might notice other dim stars among them. This “cluster” of reddish stars forming the “V” of Taurus has an official name—the Hyades—and it makes a pretty sight in a pair of binoculars. An even prettier sight is the denser cluster of blue stars above and to the right of the Hyades. This might look like a small blurry patch at a casual glance with your eyes, and in a pair of binoculars it is beautiful. This group of stars is the origin of numerous myths about sisters, it is the origin of the Subaru logo, and it is known officially as the Pleiades.

There are two bright stars at the top left of the page, close to each other, and similar in brightness, like a pair of brothers. These form one end of the constellation Gemini, and the two bright stars are Castor and Pollux, the twins.

At the top of the page is one of the brightest stars in the sky, and the most northerly bright star. It is also an excellent landmark, not only because it is bright and often up in the northern sky, but also because there are three dim stars next to it, like a badge or a nametag, making this grouping a distinctive and helpful landmark. The bright one is Capella, the Goat Star. (The word “Capella” is the diminutive of “capra”, which is the latin word for goat, and which is also the root of “Capricorn”, the sea-goat.) The three stars next to Capella are fairly dim, but being close together helps them to stand out, and they flock around Capella, like baby goats near their mother. Together they make a helpful signpost in the sky known as Capella and the Kids.

If you haven't noticed already, look at how many bright stars are present in this part of the sky. The brightest star in all the sky (if you don't count the sun) is here, along with six more in the top 20. There is Betelgeuse (#10) in the middle, and the rest form a brilliant ring around the outside. Clockwise from Sirius (#1), we find Procyon (#8), Pollux (#17), Capella (#6), Aldebaran (#14), and Rigel (#7). There is also Castor (#24) next to his brother Pollux. This gigantic ring of bright stars, each with its own bright constellation, goes by the name Winter Hexagon. (The Winter Circle is also a very common name, but personally I prefer “Winter Hexagon”.)

If you have already looked at the filled-in version or “answer key” for the worksheet, you may have noticed that the “planet highway” runs right across the Winter Hexagon, through Taurus and Gemini. (The constellations which serve as signposts marking the “planet highway” are collectively known as the “zodiac”, and the centerline of the highway is officially known as the “ecliptic”, because eclipses occur along this line.) Mars is currently approaching Taurus from the right (from Aries), and the moon will pass through Taurus over the next three days. The Pleiades and the Hyades are sometimes known as the “Golden Gate of the Ecliptic”, since they bracket the ecliptic and the moon and planets pass between them, and the moon is approaching this “Golden Gate” as I write. Watch for the moon to pass through it tonight and tomorrow, Friday and Saturday, and then to pass between the horn-tips of Taurus the next day, on Sunday.

Happy viewing!