dipper@utastro.UUCP (Debbie Byrd) (07/05/85)
Today Earth is farthest from the sun for this year. More on Earth's aphelion -- right after this. July 5 Earth at Aphelion Sensitive measuring devices aimed at the sun today could detect that we're now getting about 10 per cent less sunlight than we were six months ago. Earth's orbit around the sun isn't perfectly circular -- and today we reach the outermost part of our orbit -- so today we're farthest from the sun for this year. This annual event is called aphelion, which simply means "farthest from the sun." Earth reaches perihelion -- its closest point to the sun -- around the beginning of January each year. We go from being about 91 million miles to about 94 million miles from the sun. For people in the northern hemisphere, Earth is closest to the sun during the winter -- and farthest from the sun each summer -- so it's pretty clear that our world's distance from the sun doesn't have the major effect on whether it's hot or cold outside. Instead, the seasons we currently feel on Earth are driven almost entirely by the fact that Earth tilts by twenty-three-and-a-half degrees with respect to the plane of our orbit around the sun. If Earth didn't tilt on its axis -- but our distance from the sun still varied as it does now -- then we'd still have seasons -- caused by the 10 per cent difference in heat and light from the sun over the course of each year. So our changing distance from the sun does make a difference -- but the difference is outweighed by Earth's tilt on its axis -- which causes the northern and southern hemispheres to change places in leaning most toward the sun. Script by Deborah Byrd. (c) Copyright 1984, 1985 McDonald Observatory, University of Texas at Austin