Surprising Solstice Factsby John Rummel
Observer's Notebook for June, 2002
The summer solstice is most commonly celebrated by those of us living north of the equator as the first day of summer. The more sophisticated observer is probably aware that the day has an astronomical definition that relates to the sun. But it comes as a surprise to many that the summer solstice is really not a day at all, but a point in time. The instant of the summer solstice is defined as that moment when the sun reaches its northernmost declination for the year. We usually simplify this to say that it's the day when the sun reaches its highest elevation at noon. A stick stuck in the ground would cast the shortest shadow of the year at noon on the summer solstice.
If the Earth was not tilted, then the sun would reach the same elevation every day of the year at any given location on Earth. At the equator, it would be directly overhead at noon every day. The Earth is tilted though, at about 23 1/2 degrees. So for part of the year, the northern hemisphere of the planet is tilted toward the sun, and for the other half, the southern hemisphere is. It is the tilt that gives us the seasons, summer when our hemisphere is tilted toward the sun, winter when it's tilted away.
This year the summer solstice will occur on June 21 at exactly 8:24 am (Madison time). Since the sun will reach its highest point that day, June 21st will be the longest day of the year. The sun is above the horizon for 15 hours and 22 minutes, and below for only 8 hours and 38 minutes.
People often assume that the longest day also brings us the earliest sunrise and latest sunset of the year. Surprisingly, this is not the case. This unusual circumstance is due to the fact that the Earth's orbit around the sun is not a perfect circle, but an ellipse. As a result of this non circular orbit, the Earth is moving a bit faster in the winter (when it's closest to the sun) and a bit slower in the summer (when it's most distant). This doesn't affect the total length of the day, but it does cause the times of sunrise and sunset to vary a bit throughout the year. The size of the effect is also dependent on your location on the planet Earth.
This year, the earliest sunrise (as seen from Madison) will be on June 15th at about 5:17 am, and the latest sunset will be on June 28th at about 8:41 pm.
Incidentally, the idea that the Earth travels at variable speed as it orbits the sun is also surprising to some. You can do the calculation yourself. Grab any calendar and count off the days between the equinoxes. It takes 186 days for the Earth to travel from the Vernal Equinox (in March) to the Autumnal Equinox (in September). But it takes a week less, only 179 days, to make the return trip from the Autumnal to the next Vernal Equinox. The Earth truly does travel faster during our cold winter months.
Even though the solstice occurs in late June, it is well known that the hottest temperatures of the summer do not occur until July or August. Why is this? If the hemisphere receives the most direct and longest sunlight in June, why do the dog days not occur until much later? The effect is called the "lag of the seasons" and can be attributed to the tremendous capacity of our atmosphere and oceans to store heat. It takes a long time to warm up the hemisphere - or to cool it down - and thus our climate lags well behind the astronomical season markers.
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