Summer solstice marks change in season for astronomers
Just in case some of you may wish to stay up (or get up) and go outside to personally welcome the arrival of summer with open arms, the summer solstice — the official start to summer here in the northern hemisphere — occurs at 12:32 a.m. on June 21.
The term "solstice" comes from the Latin word "solstitium" — meaning "sol" (sun) and "stitium" (still or stopped), referring to the sun appearing to briefly stand still once it has reached its most northerly point in the sky, before beginning to move back towards its most southerly point in the sky in December (the winter solstice).
On the summer solstice (sometimes referred to as the "June solstice" after the month; or the "northern solstice" after the hemisphere in which it occurs), the Earth reaches the point in its annual orbit around the sun where the Earth's North Pole is at its maximum tilt (approximately 23.5 degrees) towards the sun, and the sun reaches its highest and northernmost point in the sky, bringing with it our much-awaited summer weather.
After reaching this position, the sun reverses its course and heads in the opposite direction (south), relative to its apparent path in the sky as viewed at the same time each day.
As the Earth orbits the sun, the rise and set positions (and times) of the sun move northward (December to June) and southward (June through December) along the horizon. The sun's apparent movement across the sky (due to Earth's axis tilt, as well as its elliptical orbital path around the sun) traces a sort of flattened figure-eight shape, referred to as an "analemma."
The date of the summer solstice varies between June 20 and 22. This is due primarily to the difference between our Gregorian calendar (365 days) and the length of a
... to a meteorologist, the summer solstice (and the beginning of the summer season) occurred June 1, while to an astronomer (and most people), the summer solstice will occur on June 21.
tropical year (the time it takes the Earth to actually complete one orbit around the sun — 365.242199 days).
To compensate for this discrepancy, we add a leap day every four years to our calendar, which makes the date of the solstice jump backward.
For example, next year's summer solstice will occur on June 21, while the solstice of 2024 will occur on June 20. The next June 22 solstice won't be until 2203 (don't think I'll make that one!).
Other influences which affect the actual date of the summer solstice include the gravitational pull of the moon and the planets, and the slight wobbling of the Earth's rotation.
Finally, there is a difference in the determination of a meteorological solstice and an astronomical solstice. Meteorologists define the seasons based on the division of the year into four, three-month periods (January to March; April to June, etc.), while astronomers divide them based on the location of the sun in the sky (solstices and equinoxes).
Thus, to a meteorologist, the summer solstice (and the beginning of the summer season) occurred June 1, while to an astronomer (and most people), the summer solstice will occur on June 21.
THIS WEEK’S SKY
While waiting for the summer solstice, let's have a look at this coming week's planets.
Mercury, having just passed inferior solar conjunction on June 11, is too close to the sun to be seen.
Venus (magnitude -3.9, look for it in the constellation Gemini — the Twins), having recently passed superior solar conjunction, is visible around 9:30 p.m. ADT, nine degrees above the northwest horizon as darkness falls. It then sinks towards the horizon, setting around 10:30 p.m. ADT.
Mars (magnitude +1.8, see it in Cancer — the Crab), heading towards superior solar conjunction in October, is visible by about 9:30 p.m. ADT, 16 degrees above the west-northwest horizon, before dropping to the horizon and setting around 11:15 p.m. ADT.
Saturn (magnitude +0.5, can be seen in Capricornus — the Sea Goat) rises just after midnight, and reaches approximately 26 degrees above the southern horizon, before fading with the approaching dawn by about 4:35 a.m. ADT.
Last to appear in the predawn sky, Jupiter (magnitude -2.5, visible in Aquarius — the Waterbearer) rises in the east-southeast sky around 12:50 a.m. ADT, reaching 30 degrees above the southern horizon, before it, too, fades from view by about 4:55 a.m. ADT.
Until next week, clear skies.
EVENTS
• June 15 — Waxing gibbous moon to upper right of Regulus (look for it in Leo the Lion)
• June 18 — First quarter moon
• June 21 — Summer solstice; 12:32 a.m.
Glenn K. Roberts lives in Stratford, P.E.I., and has been an avid amateur astronomer since he was a small child. He welcomes comments from readers at glennkroberts@ gmail.com.