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Summer’s Arrival in the Sky

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Summer’s Arrival in the Sky

Summer will officially arrive on June 21st at 6:51 a.m. EDT — the June Solstice.  At that moment, the Sun will reach that point where it is furthest north of the celestial equator. To be more precise, at the moment of the Solstice the Sun will appear to be shining directly overhead for a point on the Tropic of Cancer (latitude 23.5° north) in the Cyrenaica region of Libya.

Just as “armistice” is defined as a staying of the action of arms, “Solstice” is a staying of the Sun’s apparent motion over the latitudes of the Earth.  At the Summer Solstice, the Sun stops its northward motion and begins heading south.  At the Winter Solstice, it turns north.  Technically, even at 6:52 a.m. EDT, the Sun has turned around and started south.  It will cross the equator at the autumnal equinox, passing into the Southern Hemisphere on September 22nd, at 10:29 p.m. EDT.

All regions of the Earth north of the Tropic of Cancer can never see the Sun directly overhead, but on the day of the Solstice for cities and towns across the U.S. and Canada, the Sun will attain its highest point in the sky for this entire year.  For New Yorkers, that moment will come at 12:57 p.m. EDT when the Sun will reach its maximum altitude of 73° above the southern horizon.  Since the Sun will appear to describe such a high arc across the sky, the duration of daylight is also at its most extreme, lasting 15 hours and 4 minutes for New York.  North of the Arctic Circle (66.5° north latitude), the Sun remains above the horizon for 24-hours, the so-called “midnight Sun” effect.

Two weeks after the Solstice, at 8:00 p.m. EDT on July 3rd, Earth will reach that point in its orbit where it is farthest from the Sun in space.  Called aphelion, the Sun at that moment will be 94,506,462 miles away; Earth was closest to the Sun (called perihelion) last January 4th.

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The difference between the closest and farthest points is equal to 3,099,789 miles, which sounds like a lot, but amounts to an overall change of only 3.28-percent.  Thus this relatively small difference has very little effect on our insolation, the amount of sunlight we receive per unit of surface area.

It seems that for the Northern Hemisphere such a difference would at least tend to warm the winters and cool the summers.  The truth of the matter is, however, that the preponderance of large land masses in the Northern Hemisphere works the other way and actually tends to make the winters colder and the summers hotter.

It’s quite likely that if you ask most people in which month of the year they believe Earth is closest to the Sun, the majority would say we’re closest during June, July or August.  But our warm weather doesn’t relate to our distance from the Sun.  It’s because of the 23.5-degree tilt of the Earth’s axis that the Sun is above the horizon for different lengths of time at different seasons.  The tilt determines whether the Sun’s rays strike us at a low angle or more directly.  In summer, the Northern Hemisphere is tipped toward the Sun so that it falls more directly and is more concentrated on each unit of the surface.  And the Sun shines for a longer time each day, so it all adds up.

At New York’s latitude for instance, the more nearly direct rays at the summer solstice of June 21st brings about three times as much heat as the more slanting rays at the winter solstice on December 21st. Heat received by any region is dependent on the length of daylight and the angle of the Sun above the horizon, resulting in the differences in seasons in different parts of the world.

A final note: In 13,000 years, we northerners will tilt toward the Sun at the same time we’re closest to it.

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1 comment

1 bill reeher { 06.18.14 at 10:43 am }

your final note seems to be backwards. ?

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