To be honest, today's article has nothing to do with Pandora. We visited that jar so often last week we thought it prudent to give it a miss for a few days. Instead, we decided to compile a brief list of questions pertaining to the summer solstice. We've received this queries throughout the years from subscribers and planetarium patrons. Now that astronomical summer has begun, there seems no better time to answer these questions.
Is it true that summer is the longest season in the northern hemisphere?
Though it is difficult for northern climate dwellers to believe, summer is, indeed, the longest of the four seasons. Earth reaches aphelion (its greatest distance from the Sun) on or around July 4th. At this time, Earth's orbital velocity is at a minimum. We remember that a planet's distance from its parent star determines its orbital speed. Greater distance reduces the velocity. So, Earth requires about 92 - 93 days to travel from the summer solstice point to that of the autumnal equinox. Conversely, Earth is at perihelion (closest distance to the Sun) around January 3rd. Consequently, Earth needs only 88 - 89 days to move from the winter solstice point to that of the vernal equinox. Although winters here last a century and seem intended to punish us for a trangression a remote ancestor perpetuated, winter is the briefest of the four seasons.
Why don't we experience the "midnight sun" effect like some places do around the solstice?
We're not far enough north or south.
On the summer solstice, any region just of the Arctic Circle* (66.5 degrees N) will experience the 'midnight sun' effect. This means that the Sun will not set at these latitudes and will be visible for twenty four hours a day. On the winter (December) solstice, all regions south of the Antarctic Circle (66.5 degrees S) will experience the midnight sun effect.
These boundaries are constantly changing. Now that the solstice is over, the southern 'midnight sun' boundary will migrate north. For instance, on August 1, only observers north of 72 degrees N will have the midnight sun. On the autumal equinox, a small region centered around the north pole will have the midnight sun. (The sun would set at the north pole on the autumnal equinox were it not for our atmosphere.)
Why isn't the summer solstice on June 21st this year (2016)?
The main reason is because of our calendar. Our Gregorian calendar added a leap day in February. Had it not done so. the Sun would have reached the summer solstice on June 21st. Blame Pope Gregory XIII.
Why did the solstice Sun shift from Gemini into Taurus?
Because of precession. Earth 'wobbles' just as gyroscope does when rotating. The Sun, Moon, and -to a lesser extent- the planets induce this wobble so that the north pole describes a 47-degree circle over a 26,000 year time period. During this precessional period, the thirteen ecliptic constellations (those that the Sun appears to occupy at different times throughout the year) migrate to every part of the ecliptic. For instance, the Sun once occupied Cancer the Crab on the summer solstice. (For this reason, the latitude where the Sun passes directly overhead on the summer solstice is called the "Tropic of Cancer.") In the year 9 BC/BCE, the position of the summer solstice shifted into Gemini the twins. In 1989, this point moved into Taurus. where will it remain until moving into Aries the Ram in 4609! Even though precession is a gradual process, sometimes a seasonal point can move into a different constellation during our lifetime.
What season is it now on the Equator?
As the Equator is not a region, but a line of no thickness, there is no season on the Equator. Let's pretend you are standing on the equator facing west right now. The part of your body south of the equator is experiencing winter, the part north of the equator is in summer. Granted, you won't notice the difference.
*The boundary is just south of the Arctic Circle due to atmospheric refraction.
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FROM THE CATACOMBS OF INFINITE KNOWLEDGE
A new day begins at midnight, but "daylight" begins when the top of the Sun is first visible above the horizon.
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