PHYSICS 107. Lecture 4 Ancient Astronomy

Transcription

1 PHYSICS 107 Lecture 4 Ancient Astronomy Introduction Astronomy is the oldest science. There are astronomical artifacts going back 8000 years. Many ancient cultures on every continent seem to have had examples of sophisticated astronomical instruments, and corresponding to these instruments there must have been astronomical theories. Why was so much attention paid to astronomy even in those very early times? First of all the seasons are obviously hugely important for agricultural societies. For people living in those days it might easily be a matter of survival that crops be planted and harvested at the proper times of the year. The sun clearly has a direct positive effects on the growing of crops. That's a yearly cycle. The lunar month of about 29 days has always been thought to be connected with the human menstrual cycle, which has a similar length (modern science doesn't seem to confirm this connection). But in any case there are clear effects of astronomical objects on earthly production. These positive effects produced both astrology and astronomy, with a clear distinction being made between the two only starting in the 17th century. Another reason astronomy was important in ancient times was simply that the night sky was a much bigger part of everyday life in pre-technological societies than it is today. If you have no artificial lighting and no air conditioning you are likely to be spending a lot more time outside in the evening and night than you would today, particularly in warmer climates. Furthermore there was much less light pollution, so the background of the sky was darker and the stars and planets correspondingly brighter to the eye. These days it is very difficult to see the Milky Way even on a clear night in Madison, but far from large cities it is very visible. We ll start with some facts, some certainly familiar but other maybe not so much.

2 The sun The sun, blindingly bright, rises in the east and sets in the west. It seems to follow the same path every day, but if you watch it over the course of a year, you'll notice that the path changes. It is lower (closer to the horizon and thus further south) in the cold time of short days (winter) and higher (less far south) in the hot time of long days (summer). (This is only true in the Northern Hemisphere. In the Southern Hemisphere the sun seems to be towards the north). These different paths go along with shorter and longer days. Many of the ancient artifacts mentioned at the beginning artifacts we talk about, such as Stonehenge, clearly had as part of their purpose the determination of the summer solstice. This is the longest day of the year and it is a natural annual landmark. The moon The moon is bright enough, though it does not at all compare to the sun in this regard. It is visible on most clear nights, but sometimes, about once every 29.5 days, it disappears for a day or so. This is called a "new moon". This is not a very good name - it should be called the invisible moon. Otherwise the moon also rises in the east and sets in the west, just as the sun does. On the first night after the new moon it becomes visible. It follows the sun quite closely, rising just a bit after it and also setting just a bit after it. It is just a crescent at this time. The crescent curves towards the sun. The moon follows the sun and always stays in the same path. After the new moon, the moon gets a little fuller each day and it also gets a little further behind the sun. This is called a waxing (meaning growing ) moon. It seems to travel slightly more slowly than the sun, so it falls behind, though it still stays in just the same path as the sun. After about 14 days the moon is perfectly round, and it rises when the sun sets and it sets when the sun rises. This of course is a full moon. At this point is just the same shape and size as the sun. Then each day it gets smaller again with the curved part towards the sun (waning moon) and, after 29 days or so it gets lapped, so to speak. Then we have a new moon once more and the cycle, the lunar month, starts again. Let us summarize the motions of the moon.

3 1. Invisible. New moon. 2. Crescent shaped, not far behind the sun, with the curved part pointing toward the setting sun. Waxing moon. 3. A week after the new moon, the shape is a half circle. The moon has fallen further behind the sun, now rising at about noon. Half moon. 4. After two weeks the moon has the shape of a full circle it rises at sunset and sets at dawn. Full moon. 5. After another week we have a half-moon again. Waning moon 6. Now there are two more weeks during which the moon continues to wane, and finally disappears again and the cycle starts over. The ecliptic The sun and the moon always follow one another through the sky, traveling along the same arc. We'll also see that the planets are also always in the same arc, to a very good approximation. The reason for this is that the sun is the moon and all the rest of the planets lie in a single plane, which to us on earth than looks like an arc, since we can't see all of it. (If we could see all of it the ecliptic would be a full circle, but part of it is always below us and therefore invisible.) The plane is defined by the orbit of the earth around the sun. The moon, whose plane of motion around the Earth is very nearly, but not quite, the same as the ecliptic plane, actually crosses the ecliptic plane at periodic intervals. When this crossing coincides with a new moon, we get a solar eclipse. When it coincides with a full moon, we get a lunar eclipse. This is the reason that the plane is called the ecliptic. Eclipses Occasionally, perhaps once every 10 years, the sun is blotted out. This only happens when the moon is new, and therefore invisible. This is called a solar eclipse. It is sometimes total, meaning that the sun is completely blocked out, but usually not -- instead we have a partial eclipse in which the area of the sun is only partly obscured.

4 These solar eclipses sometimes caused panic reactions in earlier times. They were often felt to have religious significance, and many ancient peoples -- the Mayans, the Chinese, the Babylonians, -- developed means to predict them by sophisticated mathematical calculations. Solar eclipses happen when the moon is directly between the Earth and the sun. These days we also know that when a partial solar eclipse is observed somewhere on earth it is usually total somewhere else on Earth and it doesn't happen at all in some other places on earth. Lunar eclipses are considerably more common. They only occur when the moon is full: the moon becomes dimmer and reddish, one side darkens, then the whole thing. A lunar eclipse is visible in every place on earth where it is night. Lunar eclipses happen when the earth is directly between the sun and the moon. The difference in the geographic visibilities of solar and lunar eclipses, and the fact that lunar eclipses are more common, is due to the fact that the sun and the moon subtend almost the same angle when viewed from the earth, but the earth subtends a bigger angle than the sun as viewed from the moon. We would have eclipses every month if the sun and the moon were exactly in the same plane of the ecliptic. Aw I said above, it turns out that they are a little off and so usually during the period new moon the moon isn't exactly just between us and the sun, and during a full moon the earth isn't exactly directly between the moon and the sun. The stars The stars also move through the sky, but their motion is much less complicated. They move in lockstep in circles around the north star. A certain band of stars follows the same path as the sun and the moon every day. These are just the nearby stars in our galaxy that happen to be in the plane of the ecliptic. This circular band of stars, often divided into groups of stars called the constellations, is called the zodiac. The zodiac is conventionally defined to be the band of stars about 8 on either side of the ecliptic plane. The sun moves around this band once a year and the moon moves through it once every lunar month, about 27 or 28 days. Apart from their daily rotatory motion about the north star, the pattern of stars is entirely fixed.

5 The sun, the moon, and the stars all move in circular paths, the kind of motion that the ancient Greeks thought was perfect. The planets Many thousands of years ago people observed that there were exactly 5 stars that did move with respect to the other ones. They are called wondering stars, or planets. They are rather bright (except for Mercury) and they look subtly different from stars in that they don't twinkle quite as much. They seem to have some connection to the sun and moon, because they move only within the zodiac, very near the ecliptic plane that also contains the sun and moon. Mercury. Mercury is always near the sun (within 23 ), but rarely visible to the naked eye, being so near the sun. Venus. Venus is always near the sun (within 44 ) but can be very visible particularly near sunup and sundown when the sun is just below the horizon. Thus Venus is sometimes the called the Morning Star or the Evening Star. Venus can be on either side of the sun, that is above or below it, and it can also move across the sun (a transit ). Mars, Jupiter, and Saturn. These planets move through the zodiac. For the most part they move forward in the background of the stars, by which I mean they move in the same direction as the sun. But occasionally they will back up for a month or a week or so. This is called retrograde motion, which never happens with stars. Uranus and Neptune. These two behave like Mars, Jupiter and Saturn, but are only visible only with a telescope, so they are not counted in the five planets of ancient, pre-telescope, astronomy.

6 Planets really do not move in perfect circular orbits: retrograde motion was a huge exception to the perfect motion that ancient Greek astronomers thought was characteristic of the heavens. This problem would be the major issue in astronomy for the next 200 years.