Introduction to Earth Sciences I

Topic 2

Motion of the Earth in Space

What we have learned about in the first Topic are large-scale (macro) properties of the Earth. They are also mostly "static" properties - that is, properties that pertain to descriptions of the Earth, even if it were not moving in space, and even if it were internally dead. The fact that the Earth is in motion makes little if any differences to those properties. In fact, the Earth experiences a variety of complex motions in space and internally it is very much alive. These are the subjects of our next Topics.

Two aspects of the Earth's motion in space are well known to everyone. First, the Earth rotates on its axis once every 24 hours. In fact, this is what we mean by a "day". The rotation rate is not exactly 24 hours, and varies by a small amount on a daily basis; a matter of a few milliseconds. The changes are both seasonal and long term and we discuss some of them below. Additionally, it is slowing down due to tidal friction (see discussion of Tides later) so that the length of a day has actually increased over the history of the Earth. The second familiar motion is that the Earth rotates around the Sun once a year. In fact, that is what is meant by a "year".

The rotation of the Earth around the Sun in a year is marked by four seasons. The orbit is not exactly circular. It is an ellipse with the Sun at one focus. This means that sometimes the Earth is closer to the Sun than at other times. But this is not the reason why we have seasons. Remember that when it is summer in the northern hemisphere it is winter in the southern so distance from the Sun cannot be the whole answer to the season phenomenon. The reason is that the Earth is tilted so that it is not at right angles to the plane of the ecliptic (the plane in which all the planets, except Pluto, rotate around the Sun). Because of this the direct rays of the Sun fall on different parts of the Earth at different times of the year. The Earth receives the greatest heat input from the Sun when the Sun's rays are directly overhead during the daylight hours. The polar regions are cold, not because of distance from the Sun but because they always receive the Sun's energy at low slant angles so the energy gets spread over a greater area. For the same reason your shoulders get sun burnt at the beach - they present a flat upward directed surface to the Sun most of the time. You lie flat on your front to get your back tanned for the same reason.

The video below gives a good description of the cause of the seasons including a voice track.

Figure 2.0.1

Video resource: seasons video (QuickTime Video ~10MB) | seasons video (Streamed Real Media)

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