The Summer Solstice is a special moment in the grand scheme of the Earth-Sun system, a date that represents a still-stand in the cycles that govern our universe. The solstice arrives on Monday 20th for the USA (Tuesday 21st for most of the rest of the world) when the north polar axis of Earth is tilted at its maximum toward the Sun. In addition to this being the longest day in the Northern Hemisphere for the entire year, there are other interesting effects.
The duration of the longest day in the Northern Hemisphere is a relative duration. In San Francisco, the length of the day on the Solstice is 14 hours 47 minutes, but that duration depends upon the latitude north of the Equator. The further north you are located, the longer the longest-day at Solstice. For example, in Seattle the Solstice day lasts 16 hours, and in Anchorage Alaska, the Solstice day lasts 19 hours 21 minutes, and in addition when you are that far north twilight never fully fades to dark and the sky remains in twilight glow the entire time the Sun is below the horizon. Of course, extreme situations emerge in locations such as the far northern reaches of Alaska, Canada, Greenland, Iceland, the Nordic Countries and Russia that lie above the Arctic Circle where the Sun is above the horizon for 24 hours on the day of the summer Solstice.
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| Solstice & Equinox |
The term Solstice means 'Sun standing still' and refers to the perception from an Earth-bound point of view that the Sun appears to stand still in the sense of its gradual change in altitude on a daily basis. Of course, the Sun never stops moving from east to west each day, but the careful observer armed with a sundial would notice that the Sun, which has for six months been increasing in altitude at local noon each day, will appear to 'stand still' on the Solstice and gradually reverse course over the coming days, starting its six month downward trajectory toward Fall and Winter. The grand cycle of the Sun is of course attributed to the fact that the Earth is tilted on its axis and from a fixed spot on Earth, we perceive the Sun's changing altitude in a graceful cycle that slows and stops and reverses course each Solstice, and reaches a peak transition during the Equinoxes. The image shows this nicely.
This year's Solstice also features a Full Moon, so enjoy the shortest night with a bright and shiny Moon to accompany you.
This excellent article in Sky & Telescope provides further illumination on the subject.
Image courtesy Sky & Telescope.
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