 2 Night Sky Basics - Earth is our Spaceship |
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One thing that makes stargazing so interesting is that the night sky is in constant motion. Earth's spinning on its axis and its orbiting of the Sun mean that the stars in our sky are always on the move and no two nights in a year are exactly alike. In a sense, stargazing is like going on a journey in which the scenery is constantly changing.
In order to make sense of all this, here are some important-to-know Night Sky Basics.
Because of Earth's daily spinning, most stars appear to rise in the east, move 1° (one finger-width ) west every 4 minutes, and then set in the west.
The meridian is a useful imaginary line that stretches overhead from north to south. As a star moves westward, it ascends until it reaches the meridian. Then it descends. This is important because the best view of a target often occurs when it is near the meridian. There is less atmosphere to look through and so less 'twinkling' to mess up the view. The highest point of the meridian -- straight up from your location and at 90° altitude -- is called zenith.
Look north and you'll see special stars that don't rise or set -- they appear to endlessly circle Polaris, the North Star. This is because Polaris is very close to the North Celestial Pole, the point in the sky above Earth's axis of rotation. These stars are called circumpolar and the Big Dipper is a well-known example. The same circumpolar stars are visible on every clear night.
How big is the circle of circumpolar stars? It depends on your latitude. View from the equator (0°) and you won't find any circumpolar stars since Polaris would be on the northern horizon. View from the North Pole (90°) and you'll find that all stars are circumpolar since Polaris would be at zenith. A cool fact is that whatever your latitude is above the equator, that is the altitude of Polaris in your sky, day and night.
Not only are the stars in constant motion, but there is a westward progression from one night to the next. Try this: find a bright constellation in the east and note its location in the sky and the time. Then, a month later at the exact same time, do it again and you'll see that the constellation has shifted west quite a bit. Because of Earth's annual orbit of the Sun, the constellations progress westward 1° from night to night, and this adds up over time. With each season we get a new set of constellations to explore. Cool!
Are the stars actually in motion? Of course the motion of the stars is no more than a wonderful celestial illusion! The stars aren't moving, we are. Earth is our spaceship and stargazing is a great way to participate in the changing scenery.
Bon Voyage and Clear Skies!
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| Why Stargaze ◀️
▶️ Finding North |
