13 May 2008

The Days Are Getting Longer

The days are getting longer as the Earth moves in its path around the Sun. Why do the days change in length? This has to do with the axis of the Earth being tipped. Why does that affect the length of a day?

For illustration, visualize of a globe mounted on a base. Globes are always mounted at an angle, representing the fact that the Earth is tipped 23.5 degrees from vertical. That is, as the Earth spins on its axis every 24 hours, the line around which it spins points up and we call that direction North. This point in the sky is fixed over a period of hundreds of years and changes very slowly such that in a lifetime, the axis of the Earth will point nearly perfectly toward a single point in the sky. It is pure coincidence that for us in the current era, this point in the sky is almost directly where the North Star can be found. This star, Polaris, is the current north star but in a few thousand years won't be. If you want to learn more about the changing location of the pole star, you will want to read this article on "precession of the equinoxes."

As the Earth travels on its 365-day journey around the Sun, the axis of the Earth remains fixed in the same direction, North. That means that for half of the year the north side of the Earth is tipped toward the Sun, and for the next half of the year the south side of the Earth is tipped toward the Sun. That leads to the seasonal change that we all come to understand. When the north side of the Earth is tipped toward the Sun, we get more hours of light each day in the northern hemisphere. We are in that situation right now and with each passing day between now and the Summer Solstice, the days become longer. A side effect of this is that the Sun appears to rise north of due east and set north of due west. As the Sun moves across the southern sky, it climbs higher with each successive day. In the San Francisco Bay Area, it will be nearly in the zenith on the day of the Summer Solstice. This diagram illustrates how the light changes on the Earth throughout the year. Enjoy the sunshine and warm weather and take note of how high the Sun climbs in the sky at midday.

Note: Mercury remains visible for a few more days and is changing phase rapidly. It is found in the north-west part of the sky just after sunset.

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