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We’re looking at the question, "why are the days getting longer?"
Remember! This is a reprint of an article that originally appeared in the LA Times on May 20, 1999. That means the days were getting longer as summer approached. Now summer is over and winter is approaching. Have you noticed how the length of day has been changing?
We have collected data on the changing day length and last week we saw that when the days are getting longer in Los Angeles, they are actually getting shorter in Brazil. But the question still is "Why does day length change?"
People have always been interested in understanding the Sun’s relation to the Earth. The length of the day has important effects on our weather, our seasons, our harvests
, and on how we think about the world around us. Throughout
history, there have been two main models to explain the seasonal changes and the
length of the day: heliocentric (with the Sun/Helios in the center of the plan
etary system) and geocentric (with the Earth/Geos at the center).
The illustration below shows the heliocentric model as proposed by Copernicus in the 1540's. In this model, the change in day length is related to the tilt of the Earth as it move
s around the Sun. When the Northern Hemisphere is tilting towards the Sun, the day is long and we have summer. Note that when Los Angeles is tilted towards the Sun, locations in the Southern Hemisphere (such as in Brazil) are titled away from the Sun, so the length of the day there is shorter. In each case, the change in day length results from a change in the angle of the Earth in relation to the
Sun.
What I would like you to do for next week is to tell me how YOU would
decide which model is correct. What experiment would you do to figure out whether the Sun goes around the Earth, or the Earth goes around the Sun? Can you tell the difference from measuring the movement of the Sun alone? If the Earth is actually
moving around the Sun (as most people now believe), why is it that the Sun (and
not the Earth) appears to be moving across the sky? Next week we will publish
your answers to these questions, so send them in or post your answers at the bulletin boards at the Sun Spot!
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