|Wed, 21 Jun 2017 at||00:15 EDT||(7 days ago)|
Dominic Ford, Editor
From the Earth feed
21 June will be the longest day of 2017 in the northern hemisphere, midsummer day.
This is the day of the year when the Sun's annual passage through the constellations of the zodiac carries it to its most northerly point in the sky, in the constellation of Cancer at a declination of 23.5°N.
On this day, the Sun is above the horizon for the longer than on any other day of the year in the northern hemisphere. This is counted by astronomers to be the first day of summer, though meteorologists often take summer to start on June 1.
Conversely, in the southern hemisphere, the Sun is above the horizon for less time than on any other day of the year and astronomers define this day to be the first day of winter.
At the June solstice, the Sun appears overhead at noon when observed from locations on the tropic of Cancer, at a latitude 23.5°N.
This fact was used by the ancient Greek astronomer Eratrosthenes in around 200 BC to work out the radius of the Earth for the first time. He knew that at midsummer, the Sun appeared exactly overhead in the Egyptian city of Swenet (now Aswan), because its light shone right down to the bottom of deep wells.
He travelled to Alexandria, on the Egyptian north coast, at a distance of 5,000 stades from Swenet. Here, he used a stick in the ground to determine that the Sun was seven degrees away from the zenith at midsummer, implying that a distance of 5,000 stades around the circumference of the Earth corresponded to a distance of seven degrees around the Earth's curved surface.
Thanks to this experiment, the ancient Greeks were well aware that the Earth was spherical and had a very good idea exactly how big it was, long before anyone had circumnavigated the globe.
The exact position of the Sun when it reaches its most southerly declination in 2017 will be:
|Object||Right Ascension||Declination||Constellation||Angular Size|
The coordinates above are given in J2000.0.
|The sky on 21 June 2017|
All times shown in EDT.
The circumstances of this event were computed using the DE405 planetary ephemeris published by the Jet Propulsion Laboratory (JPL).
This event was automatically generated by searching the ephemeris for planetary alignments which are of interest to amateur astronomers, and the text above was generated based on an estimate of your location.
© NASA/Apollo 17