Summer Solstice 2017

Southern-facing view for US eastern seaboard, mid-northern latitudes for this year’s Summer Solstice. In this view, note the position of the planets Mercury, very close to the sun, Mars, Venus and the Moon, our natural satellite. View via Stellarium.

During the yearly journey around our star, the Earth passes four notable points, points that mark the changing seasons, the Vernal Equinox, the Summer Solstice, the Autumnal Equinox and the Winter Solstice. Tomorrow, 21 June at 00:24, EDT, the sun will appear at it’s highest elevation above the southern horizon for points in the Northern Hemisphere and thus, will mark this year’s Summer Solstice, the astronomical beginning of Summer. Given that the position of the sun is a result of the earth’s tilt and the observer’s location on its surface, the Summer Solstice is also when the sun will appear at the Zenith, the point directly overhead, for observers at the Tropic of Cancer (23.5° north latitude). Six month’s hence, the sun will appear at it’s highest location at the Tropic of Capricorn (23.5° south latitude) for observers in the Southern Hemisphere or, alternatively, during the Winter Solstice, here in the Northern Hemisphere. The Summer Solstice also marks the longest day and shortest night of the year for observers in the Northern Hemisphere.

It is this, the elevation of the sun above the horizon, or the “angle of insolation”, that causes the seasonal warming and cooling. Consider the following thought experiment: hold a flashlight directly above the floor, vertically and pointing directly “down” at the floor; this position would be the analog of the Summer Solstice at the Tropic of Cancer. Note that the beam changes concentration and becomes more or less elongated as you vary the flashlight’s angle. If you were to measure the energy density of the beam on the floor, you would note that it would decrease as the angle decreased, approaching zero for a flashlight parallel to the floor. It is this, the angle of insolation, that causes warming during the summer in the northern hemisphere, not the earth’s distance from the sun. In fact, the earth is at its aphelion, it’s furthest distance from the sun, during the month of July!

Conversely, the sun’s position in the sky and the corresponding seasons are reversed for observers in the southern hemisphere. Our friends in Australia are, perhaps, enjoying their “winter” holidays (Christmas, etc) vacationing at the beach!

Illustration showing the Earth’s annual journey around our star. Note that our planet’s axis of rotation is always pointing in the same direction towards the North Celestial Pole, the extension of the Earth’s physical pole on the sky. Image credit via (Tauʻolunga).

I highly recommend the Open Source application, Stellarium, a fully-configurable, digital planetarium for your desktop or mobile phone.


All our science, measured against reality, is primitive and childlike-and yet it is the most precious thing we have

An index of all articles in this blog can be found here.

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