The Night Sky |
(Think about these carefully before you consult the answers below)
QUESTIONS
1) In the Northern Hemisphere, the Sun, Moon and planets all rise in the East and set in the West. What happens in the Southern Hemisphere? <answer>
2) What characteristics do the stars in the same constellation share? <answer>
3) What would the constellations look like from Mars? What would they look like from a planet located on the other side of our Milky Way Galaxy? <answer>
4) Both astronomers and astrologers talk of constellations of the Zodiac. What is the significance to each? <answer>
5) The Earth turns eastward at approximately 15 degrees per hour, causing the stars to appear to move westward at about 15 degrees per hour, which you can note in the activity. Thus in 24 hours the stars appear to move about 360 degrees, and return to about the same place at the same time as the night before. Actually, however, the Earth turns at just slightly more than 15 degrees per hour, and the stars appear to move at just slightly more than 15 degrees per hour. The result is that the stars arrive at the same position in the sky 4 minutes earlier each night (23 hours 56 minutes rather than 24 hours). What is the significance of this? <answer>
ANSWERS
1) The Earth does not change its direction of rotation, so the Sun, Moon, stars and planets all rise in the East and set in the West, regardless of whether you are in the Northern or Southern Hemispheres. However, in the Northern Hemisphere, the path of the Sun, Moon and planets (the Ecliptic), crosses the southern sky. Hence popular star charts are normally aligned with South at the bottom and North at the top. The north/south directions are reversed in the Southern Hemisphere, and the Sun, Moon and planets take paths across the northern half of the sky.
2) In general, the stars in constellations share no specific characteristics except those that are common to stars in general, and the fact that they are in the same general direction as seen from Earth. Two stars that appear quite close together and of similar brightness in a constellation could be completely different spectral types, and one could be 10s to 100s of times farther away than the other. Except perhaps in rare cases, they bear no specific relationship to each other at all.
3) Most of the stars in constellations are relatively speaking close to us compared to other stars in the Galaxy. However, they are still many trillions of miles away. Mars is thousands of times closer to us, and hence the appearance of the constellations is essentially the same as on Earth. It's a bit like viewing an island straight ahead on the horizon from a cruise ship. The view you see from the port (left) side of the ship is essentially the same as the view a friend might have from the starboard (right) side.
4) To astronomers, constellations of the Zodiac (that is, those through which the Sun and planets move through the year), have no particular significance. Astrologers, on the other hand, attribute various "influences" on the zodiacal constellations as well as to the celestial objects that move through them. However, of the four known physical forces in the Universe, none has been shown to have the kind of influence over life on Earth that astrology claims. The existence of any other "influence" has not been demonstrated, nor does it even appear likely from a scientific viewpoint.
5) Our clocks keep ordinary time with a 24-hour day.Originally this was intended to represent the average time of the Earth's rotation with respect to the Sun (known as a "mean solar day"). The average time of the Earth's rotation with the distant stars, however, is about 23 hours, 56 minutes -- some 4 minutes short of the 24-hour period. This difference is due to the Earth's motion around the Sun. During one day the Earth moves along its orbit around the Sun, and hence our position relative to the stars changes slightly from night to night. The result is that it takes 4 minutes less time to rotate relative to the stars than it does to rotate relative to the Sun, on the average.
So what significance is this? Not that 4 minutes per day amounts to about 120 minutes, or two hours per 30-day month. Hence if a star or constellation rises at 10 p.m. tonight, it will rise at 8 p.m. in a month, and 6 p.m. in two months (disregarding any complications of Daylight Time). It should be obvious, then, that if a star rises tonight at midnight, six months from now, it will rise 12 hours earlier. As noted earlier, one hour of time amounts to about 15 degrees of rotation. Thus 12 hours is about 180 degrees. So a constellation rising at midnight tonight will be setting at midnight in about 6 months. After 12 months, it will be back at the same place at the same time and date. This is why we speak of certain constellations as being Winter or Summer constellations -- because they are visible in the evening sky in those seasons.
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