Summer solstice

Okay, here comes some sciency stuff.

Summer solstice happens twice a year—once in the Northern Hemisphere (June 21) and once in the Southern Hemisphere (December 22). Those are the two days when the Sun spends the most time in the sky.

If you have a globe handy, you’ll see that there’s a line that goes all the way around the middle of the Earth. That’s the equator. It separates the Earth into a northern half and a southern half. Each half is called a hemisphere—’hemi’ means ‘half’ and ‘sphere’ is another word for ‘globe’.

The Sun appears to travel across the sky, but ackshually the Earth is traveling around the Sun and spinning on her axis at the same time. An axis is the line that goes through the center of the Earth from the North Pole to the South Pole. Did you notice that your globe’s axis is tilted? It’s tilted at a little over 27 degrees, because Earth is tilted at a little over 27° in relation to its orbit around the Sun. Because of this tilt, half the year (the time it takes to orbit the Sun) the Northern Hemisphere receives more of the Sun’s rays. The other half of the year, the Southern Hemisphere receives more of the Sun’s rays. The equator gets the Sun’s rays all year round, so it’s always hot there.

It takes one year for Earth to orbit the Sun. When the Northern Hemisphere is tilted toward the Sun it’s Summer there. When the Southern Hemisphere is tilted toward the Sun it’s Summer there.

Getting back to that equator—let’s look at the globe again. There are 2 other lines that also run around the Earth. In the Northern Hemisphere, there’s the Tropic of Cancer. In the Southern Hemisphere, there’s the Tropic of Capricorn. On June 21, the Northern Hemisphere tilts toward the Sun and the Tropic of Cancer gets the Sun’s rays directly. You could draw a line from the Earth’s center through the Tropic of Cancer to the Sun. On December 22, the Southern Hemisphere tilts toward the Sun and the Tropic of Capricorn gets the Sun’s rays directly. You could draw a line from the Earth’s center through the Tropic of Capricorn to the Sun.

On June 21, the Northern Hemisphere has the most daylight of any day of the year. Six months later, on December 22, the Southern Hemisphere has the most daylight of any day of the year.

Remember, when the Northern Hemisphere has the most hours of daylight (Summer solstice) the Southern Hemisphere has the fewest hours of daylight (Winter solstice). The opposite is true at solstice six months later.

Three months after solstice is the day when the day is split equally in two, with 12 hours of daylight and 12 hours of nighttime. This is the equinox. The autumnal (Fall) equinox is September 22 in the Northern Hemisphere. March 21 is the vernal (Spring) equinox.

Why am I telling you this? Well, over two thousand years ago a Greek librarian figured out exactly how big around the Earth is using only a well and a stick—and he did it at noon during the Summer solstice.

Back to the beginning of The Western Civ User’s Guide to Time & Space