As in Philosophy, so it is in Astronomy, there are many events that happen on a regular basis that we take these proceedings for granted. In Philosophy the question, “How do you know that the sun will rise again tomorrow?” always comes across the first year philosophy student and leaves him just as perplexed about it at the end of his ponderings as he did prior to them. The fact of the matter is, the sun does not move in the manner in which we commonly take for granted.

Any movement of the sun plays no part on what we on Earth call night and day. The Earth, as most of us learn in fourth or fifth grade, spins on an axis. This spin is called rotation. The Earth’s rotational axis is tilted at 23.4° and spins at about 1038 miles per hour. This rotation occurs in a counter-clockwise motion (if looking down upon the northern axis) which gives one the illusion of the sun rising in the East and setting on the Western horizon. In the space of a calendar day a person, just standing in one spot travels about 24,912 miles depending where on the Earth you are standing. If a person is standing at either of the two axis points called the North and South Poles, the speed of travel would be zero. Through out ancient times, men of science believed that due to the Earth’s weight alone (that it was too heavy) was proof that the Sun revolved around the Earth.

In 1530, Polish Astronomer Mikolaj Kopernik or better known as Nicolaus Copernicus explained in his work “De Revolutionibus” that it was the Earth that spun on it’s axis on a daily basis and that it was the Earth revolved around the Sun every year. It takes the Earth 365.24 days to makes this revolution around the sun. All the planets in the Sol solar system revolve around the sun. Mercury takes about 88 Earth days to revolve around the sun due to its relative proximity to the Sun. Venus takes a little longer, 266 days to makes its journey. The Earth, like some of the other planets in this solar system travel in not quite a circular pattern around the sun, the pattern is actually an ellipse.

The fact that the Earth travels in an elliptical pattern around the sun led many to believe that this was the cause of the seasons. Due to the slight difference in distance that the Earth would be in relationship to the sun at certain points one would think that when the Earth was at its closest point (perihelion) to the sun this would the in the summer. In actuality this perihelion actually occurs on or around January 2nd, in the midst of winter and the Earth is at this time is only 91.4 million miles from the sun. At aphelion, the Earth is the farthest from the sun at 94.5 million miles and this occurs around July 3rd. The Earth’s tilt on its axis of 23.4° is the actual cause of the seasons and not the proximity of the Earth to the Sun. The hemisphere that is tilted toward the sun at that particular time of the year will receive more daylight (longer days) and this is the time called summer. It takes the Earth about 3 months to move from one Equinox to the next.

In summing all this spinning up: The Earth rotates around an axis once every 24 hours; The Earth revolves around the sun once every year. The Sun revolves around the Milky Way galaxy once every 230 million years. Finally the Milky Way is moving towards the Andromeda galaxy which is also moving towards the Virgo Cluster of galaxies.

Some Great Links:

Sun’s movement around the galaxy
http://curious.astro.cornell.edu/question.php?number=96

Copernicus
http://www-history.mcs.st-andrews.ac.uk/~history/Biographies/Copernicus.html
http://www.blupete.com/Literature/Biographies/Science/Copernicus.htm

Tilt
http://en.wikipedia.org/wiki/Obliquity