March 29, 2010

Pathways in the Spring Skies


Part of each of my “star party” presentations is what I call “constellation tours”. One of the most well known constellations is Ursa Major the “Great Bear” or “Big Dipper”. The “Dipper” is one of the easiest constellations to spot and it never sets for those of us here in the Ohio Valley. If you know the Dipper you’ll never be lost and it serves as a guidepost to several other constallations of late winter and early spring. 

Before we start looking for constellations here’s a hint on how you can determine angular distances in the sky using “hands and fists”. Whether you’re 4 or 94 you can use your hands and fists as “yardsticks” to measure the angular separation between stars, planets and other celestial objects. Here’s how: fully extend one arm. At arm’s length one finger will subtend, or cover, about 1-2 degrees of the sky. A fist at arm’s length is about 10 degrees wide including the thumb and, with your hand open and fingers extended, about 20 degrees from pinkie to thumb. You should be able to “measure” two stars 30 degrees apart with three fists at arms length. You can test your “celestial fist yardstick” by measuring from the horizon the point directly overhead which is know as “zenith”. Zenith is 90 degrees above the horizon so try making this measurement by “stacking” and counting fists. Horizon to Zenith should be 9 fists. Got it?

On a clear evening step outside as darkness falls. In late March our sun sets at approximately 8 pm so I suggest you try this around 8:30 pm. If you need to get your bearings face the direction of the sunset, which is west. South will be to your left, north to your right. Do a 90 degree “right face” and you will be facing North.

On early Spring evenings the “Dipper” will be seen standing on its handle to the northeast with the “bowl” facing west. Imagine the northern, or circumpolar, region of the sky as the face of a giant clock. During early Spring evenings the Dipper appears at about the 2 o’clock position on our imaginary clock’s face. Can you spot it? The “Dipper” is the visually most obvious seven (actually eight) stars in the Ursa Major constellation. The “scoop” part of the Dipper consists of 4 stars. This time of year the scoop is open on it’s left or western side. The handle consists of 3 stars which form an arc that slopes down toward the Northeast.

Once you’ve located the Big Dipper certain combinations of its stars can be used to direct us to other stars and constellations. The “front” stars on the Dipper’s bowl are named Dubhe and Merak and are known as the “Pointer” stars because they always point to Polaris, the “North Star”. The “Pointers” are about 5 degrees apart (roughly three fingers or half a fist). Trace an imaginary line to the left, or westward, about three fists or 30 degrees until you come to Polaris which isn’t a particularily bright star but is the brightest one in that area of the sky. Polaris isn’t exactly on the celestial north pole but it’s within less than a degree of it so we can call it our North Star.

Polaris is part of Ursa Minor, the “Little Dipper”. The Pole Star is at the end of the Little Dipper’s handle and here in early Spring Ursa Minor is lying on it’s back stretching eastward to the right of Polaris. Unless you’re observing from a fairly dark location you’ll probably only be able to see the two brighter stars on the front of the Little Dipper’s bowl. The two Dippers are always complimenatary, that is, if filled with water one of them will empty into the other. In the Spring imagine the Big Dipper pouring its water into the Little Dipper below. The opposite will be true in Fall, six months hence.

Earlier I noted that the Big Dipper is actually eight stars. Look closely at the Dipper’s handle and focus on the second star from the handle’s end. This is actually the double star pair consisting of brighter Mizar and dimmer Alcor. Can you separate them? Some Native American cultures used the Mizar-Alcor pair to test visual acuity. Follow the curve of the Dipper’s handle down about 30 degrees northeast to the bright red-orange star Arcturus in the constellation Bootes, the Herdsman. If you can follow the same arc another 30 degrees (three more fists) without hitting the horizon you’ll come to the white star Spica in constellation Virgo. An easy way to remember this pathway is “arc to Arcturus” and “spike on to Spica”.

Back to the Dipper’s Pointers once more but this time to the south and east in the direction away from Polaris. Trace an imaginary line off the backside of the Pointers southward for about 35 degrees until you reach the blue-white star Regulus, just south of Zenith. Regulus is the twenty-first brightest star in the sky and sits at the base of a “reverse question mark” called the Sickle. Regulus is the heart of Leo, the Lion. The Sickle forms Leo’s mane. A triangle of stars to Regulus’ east marks Leo’s body. Leo is one of the most easily discerned constellations. Can you spot the Lion?

The phrase “March comes in like a Lion, and ends like a lamb” probably originated because on evenings in March the constellation Leo the Lion is rising prominently in the southeastern sky.

Remember, when you face Polaris and the Big Dipper you’re looking North. With this knowledge of the night sky you should always be able to find your way.

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