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If you have dark skies, you've probably seen stars that are thousands of light-years away. Others are less than 10 light-years away. A light-year is the distance light travels in one year, moving at 186,000 miles per second. Our Milky Way Galaxy is more than 100,000 light-years across. It contains billions of stars, but the most adept skywatchers can see no more than 6,000 of them without a telescope. Skywatching can teach you about celestial mechanics. By watching the movement of the planets and stars between, say, dinner and bedtime, you can observ how the Earth spins on its axis. The Moon, planets, and stars traverse the night sky (and the Sun the day sky) from east to west because the Earth rotates from west to east. By noting the changing positions of stars and planets over the course of two or more nights, you can witness how the Earth's yearly motion around the Sun alters the positions of objects in the night sky. Stars near the sky's equator rise (and set) four minutes earlier each night. There's some nifty magic to the math here: Stars rise (and set) two hours earlier each month, and 24 hours earlier each year - when the Earth has completed one orbit around the Sun. The four-minute rule works for the planets Jupiter and Saturn, and often for Mars, Mosley says. Mercury and Venus orbit the Sun relatively quickly, though, and their paths are more complicated. Stars near the sky's north pole remain above the horizon throughout the year, and the North Star remains in the same position - above the North Pole of the Earth. |
