Question

What 2 constellations or clusters of stars can we see all year long?

Asked by: Helen

Answer

It depends on where you live, specifically your latitude, on the Earth. At the North Pole (90 deg latitude), for example, the North Star is directly overhead and any stars within 90 degrees of it are visible year around. At the Equator (0 deg latitude), however, the North Star is on the horizon and only stars withing 0 degrees of it (ie. no stars) are visible year around.

In the more likely situation that you live around 45 degrees latitude, the North Star would appear 45 degrees above the horizon and only stars within 45 degrees of it are visible throughout the year. This would include constellations like Ursa Major (the Big Dipper), Ursa Minor (the Little Dipper), Cassiopeia, Draco, and other so-called "circumpolar" constellations.

Answered by: Paul Walorski, B.A., Part-time Physics Instructor


Answer

It depends completely on where you live.

EXPLANATION:

To understand the answer to this question, you have to understand how Earth is situated in space. Earth's axis is tilted, but it always points the same direction. That direction happens to be straight at a bright star named Polaris, which is commonly called the North Star. Earth revolves once a day, and where it is in that revolution when the sun sets changes throughout the year. That is why the constellations you see at night change.

If you lived at the North Pole, the North Star would be directly overhead, and the constellations would wheel around you like the horses in a giant carousel would if you stood in the middle. You would only see half of the constellations, but the same ones would be in the sky all the time; none of them would ever set. However, for some parts of the year the sun never sets either! And that's no good for seeing constellations.

If you lived on the equator, it would be completely different. The giant carousel would be on its side. The North Star would sit right on the northern horizon. Each constellation would spend half of its time in the sky. Each one would come up in the east and set in the west 12 hours later. However, each day, the sun would blot out half of them, and the ones it blotted out would change throughout the year. Once again, you couldn't see any of them all the time.

Most people live in between those two extremes. Halfway between the equator and the North Pole, half of the sky acts like it does from the equator, and half of it looks like it does from the pole. The giant carousel is tilted halfway up, and the North Star is halfway up the sky. The stars opposite the pole rise and set each day, but the stars near the pole (and the North Star) go in circles. If one goes in a small enough circle, it will be up all night long, every night. (We would call that star "circumpolar".) If the circle gets too big, the star will be cut off by the horizon some of the time.

ANSWER:

The number of constellations that are up all year increases as you move farther north, so you can't get an exact answer without submitting your address, but here are the constellations closest to the North Star: Ursa Minor (the Little Dipper, which contains the North Star), Ursa Major (which the Big Dipper or Plough is part of), Draco, Cepheus, Cassiopeia, and the very faint Lynx and Camelopardalis. The Big Dipper and Cassiopeia are both close and distinct, so they are a good bet for your two star patterns.

This all works the same for people in the Southern Hemisphere except that there isn't a bright South Star. The point where it would be if it existed is called the South Celestial Pole. The constellations closest to this pole are: Octans, Apus, Triangulum Australe, Musca, Chameleon, Volans, Mensa, Reticulum, Hydrus, Tucana, and Pavo. (They tend to be smaller than their northern counterparts, so there are more of them.) Octans and Chameleon are the two closest, but none of these patterns are very distinct.

Answered by: Dan Gerhards, B.A., Instructor at Sean's Astronomy Shop, WA, USA

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