| READING | PRACTICE: from Prologue, pg.15 |
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They are due to the rotation of the Earth about its axis. As the earth rotates, different location come into the sunlight, while other locations go in the opposite direction from the Sun, into the darkness.
The seasons come about because of the different amount of heat that the northern and southern hemisphere receive as the Earth goes around the Sun. In the summer and winter the Earth is on opposite sides of the sun. As the above sketch shows, the tilt of the Earth's axis positions the northern hemisphere in the summer so that it faces towards the Sun and receives rays from the Sun directly down (vertically). Thus heating is very effective. In the winter the Sun's rays fall on the northern hemisphere obliquely and heating is not effective. This is because the same amount of energy come from the Sun is spread out over a larger area, thus any specific location on the Earth receives less heat compared to the case of vertical illumination (see Fig.P.8 in the Prolog in textbook).
The sketch below shows how sunlight illuminates the Earth during the month of July and how this results in day-time on one side and night-time on the other. The red lines show how an location on the surface of the Earth rotates moves as the Earth rotates. This figure is the basis for explaining why in the northern hemisphere days are longer during the summer and shorter during the winter. The full details will be explained in the next lecture.
The reason for having longer days in the summer than in the winter is also related to the tilt of the Earth's axis. The above sketch shows how sunlight illuminates the Earth during the summer and results in day-time on one side and night-time on the other. The Earth's rotation axis is tilted relative to the day/night line. As the Earth rotates, a fixed location on the surface of the Earth follows a track similar to the ones shown by the red lines.
From the relative lengths of the tracks we can see that a location in the northern hemisphere spends more time in the day-time than in the night-time and vice versa for the southern hemisphere. A location at the Earth's equator has equal days and nights all year round. And at the Earth's north pole the sun never sets during the summer. Similarly in the south pole the sun never rises in the summer. The situation reverses in the winter.
The dates on which the length of the day is the longest or the shortest have special names. Here are some definitions worth remembering:
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June 21 |
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December 21 |
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March 21 |
have equal lengths |
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September 21 |
The stars are fixed relative to the sun, while the Earth is not. We cannot see stars in the direction of the sun. Therefore, Orion, for example, is not visible in the summer (in the direction of the sun) but only in the winter.
The motion of the Sun in the sky over the course of a year does not follow the Earth's equator. Rather it follows a circle that is tilted relative to the Earth's equator. This path is called the ecliptic (illustrated in Fig.P.7 in the Prolog in the textbook); it takes the Sun towards the north in the summer and towards the south during the winter. The 12 constellations that happen to lie along the ecliptic (i.e., follow a circular path similar to the Sun's) are called the the Zodiac (see Fig. P.6 in the Prolog in the textbook).
These constellations are associated with one's astrological birth sign as follows: one's sign is the constellation that falls in the direction of the Sun on the date that they were born. For example, if one was born is late April, the chart in the above figure indicates that the Sun was in Taurus, therefore their sign is Taurus. However, one should remember that astrological charts are outdates; they are based on the path of the Sun through the Zodiac as it was in the year 131 B.C. Since then, the path of the Sun has actually changed because the direction of the Earths spin axis has shifted. The Earth's spin axis precesses like a spinning top (see Fig.P.9 in The Prolog in the textbook) and completes a precession cycle in approximately 26,000 years. This phenomenon is called the precession of the equinoxes. Therefore in the 2,100 years since the astrological charts were devised the Sun's path has moved over by one constellation.
See Fig.1.1 in Chapter 1 of the textbook. New Moon occurs when the moon is on the same side of the Earth as the Sun. Full Moon occurs when the Moon is on the opposite side of the Earth from the Sun. The phases of the Moon are the result of the different directions relative to the Earth from which the Sun's light hits the Moon. These relative directions change as the Moon goes about its orbit: the direction of the Sun's light stays roughly the same over one lunar orbit but the viewing direction of the Moon from the Earth changes.
The nearly-circular paths defined by the Moon's motion around the Earth and by the Earth's motion around the Sun do not line up in the same plane. As seen from the Earth, the Sun orbits in the ecliptic plane, while the moons orbital plane is slightly tilted relative to the ecliptic (see Fig.1.5. in Chapter 1 of the textbook).
The Earth, Sun, and Moon line up twice in a lunar orbit (Sun-Earth-Moon, Sun-Moon-Earth) but the alignment is rarely exact because the path from the Moon is not exactly in the plane of the ecliptic. In those rare cases when the alignment is very close, we get an eclipse of the Moon or the Sun.
Total Lunar Eclipse: occurs because the Earth's shadow falls on the moon. The whole moon is covered. It can only happen during a full moon because the moon has to be lined up behind the Earth, in the opposite direction from the Sun (see Fig.1.2. in Chapter 1 of the textbook).
Partial Lunar Eclipse: imperfect alignment of the Earth's shadow with the moon; only part of the moon is covered.
Solar Eclipse: the moon passes in front of the Sun. What is the phase of the moon during a solar eclipse? It must be a new moon because that is when the Moon is properly lines up between the Earth and the Sun.
Annular Eclipse: the moon is further away from the Earth than average (because of its elliptical orbit), so it does not cover the entire Sun during the eclipse: it covers the center and leaves a ring of light around it (see Fig.1.4. in Chapter 1 of the textbook).
Eclipses are rare: they can only occur when the moon crosses the Earth's plane of revolution around the Sun at the same time as it is lined up either between the Earth and the Sun (Solar eclipse) or behind the Earth (Lunar eclipse).
Planets are "wanderers" - they do not follow yearly cycles. Stars rise and set at the same time and at the same place on a given day of the year. Planets, on the other hand, can be found at different locations in the sky relative to the stars. On different nights they shift their location, appearing to drift or wander relative to the stars. Most of the time the planets drift from East to West relative to the stars but sometimes their motion relative to the stars becomes very strange: the reverse the direction in which they drift and for a while they appear to be moving "backwards" relative to their original direction, i.e., West to East; this phenomenon is called retrograde motion (see Fig. 1.7 in Chapter 1 of the textbook). They also exhibit other unusual characteristics, not seen in the stars: they appear to vary in brightness, and a couple of them, Mercury and Venus, show phases, just like the phases of the Moon. Mercury and Venus are only visible just after sunset or just before sunrise.
You can find detailed information about the motion of the planets each season, and on how to observe them in a couple of popular astronomy magazines: Astronomy and Sky and Telescope. These magazines have web sites which you can visit by following the above links.
For a very long time the commonly held view was that the Earth was at the center of the Universe and everything in the sky was orbiting around it, including the Sun, the stars, and the planets. This is known today as the Geocentric model of the Solar System (i.e., centered on the Earth; see Figs.P2 and P.4 in the Prolog in the textbook). It was originally suggested by Aristotle in the 4th century B.C. and was developed into a proper mathematical model by Ptolemy in the 2nd century B.C. In this framework, the motion of the Sun and the stars was easily explained in an aesthetically appealing way: they were going around the Earth in circles.
The motion and other aspects of the behavior of the planets posed a considerable challenge for the Geocentric model of the Solar System because they did not seem to be moving in a regular fashion relative to the distant stars. For this reason, Ptolemy had to devise an elaborate system of epicycles (i.e., circles upon circles) to describe their motion (see Figs.1.8 and 1.9 in Chapter 1 of the textbook). The planets had to execute epicyclic motion rather than simple circular motion for us to see them move as they do from our point of view (on the Earth). The Geocentric model for the solar system thus became fairly complicated and started to lose its aesthetic appeal. It was nevertheless embraced by everyone until the renaissance when the simpler (hence aesthetically more appealing) Heliocentric model (i.e., centered on the Sun) was developed. This models is considered next, in Lecture 4.