How is the instant the seasons change determined?
Hello Professor Stern,
I came along to your site regarding seasons while researching a question
- How do scientists approximate the time (down to the minute) that
seasons change? Can you answer this question or point me in the right
direction to find the answer.
Reply
Dear Christiana
During the year, the Sun seems to follow a large circle around the sky, among the "fixed" stars which define the constellations and whose positions are essentially fixed, because they are so far. That circle is known as the ecliptic, and is described on the web site.
Because of the Earth's rotation, the sphere of stars also seems to rotate. It has two "poles" around which the rotation takes place, one in the northern half of the sky and the other in the southern half. The first is visible only north of the equator, the second only south of it. The line halfway between the poles is knows as the equator of the celestial sphere, or the celestial equator. This, too, is described in the first sections of "Stargazers".
These two circles--the ecliptic and the celestial equator--intersect (as any two circles on a sphere must do, if each divides the sphere in two equal halves). The seasons are defined somewhat arbitrarily by this geometry. The two points of intersection are the spring and fall equinox, and when the Sun is there, night and day are equal (hence the name). The first is viewed as the start of spring,: see story on the Persian New Year in the section on the calendar, in the yellow box there. The second is the start of fall, around September 23.
Summer starts when the Sun is as far north of the ecliptic as it can get--that is the summer solstice, around June 21, the longest day. And winter starts when the Sun is as far south as it can get, and that is when nights are longest, around December 21.
Of course, we can't observe the Sun's position among the stars. But we can infer it, say by noting the times when on two consecutive days it passes exactly to the south. Using an accurate clock and observing the st