How Come?

How come eggs can balance on end on the first day of Spring? asks a reader.

It’s coming: this Thursday, March 20^th, at 1:48 a.m. EDT. You may sleep through the Vernal Equinox, but when you wake up, it will be the first day of Spring in the Northern Hemisphere.

The Vernal or Spring Equinox is the midpoint between the Winter Solstice in December, when the hours of daylight begin increasing, and the Summer Solstice in June, when the days begin to grow shorter again. Each day, the sun stays in the sky about two minutes longer, rising a little earlier and setting a little later. Until, near June 20th, we’re enjoying the longest days of the year.

At the moment we call the Spring Equinox, an observer on the Equator will see the sun’s center pass directly overhead. (He’ll notice the same thing during the Fall or Autumnal Equinox, in September.) Meanwhile, at the North Pole, an observer will see the Sun hugging the horizon: Spring Equinox is opening day for six months of daylight at the top of the world.

The equinoxes are a result of the Earth’s tilted journey around the Sun. Our planet rides through space sideways, tipped at about a 23-degreee angle. So the North Pole is leaning toward the Sun for half the year, away from the Sun for the rest.

But on March 20^th (and again on September 22nd, 2008), the North and South Poles will be about equally distant from the sun. Since the Sun is shining directly on the equator rather than at an angle, the Earth is bathed, half and half, in darkness and light.

In fact, the word “equinox” comes from Latin words meaning “equal night.” But the first day and night of spring aren’t exactly 12 hours each. The Sun as seen from Earth is a disc, and the day begins and ends when the top edge of the disc rises or sinks below the horizon. But the Earth’s pesky atmosphere bends (refracts) incoming sunlight, making the Sun appear to rise sooner, and set later, than it actually does. So on the Equinox, day actually lasts a few minutes longer than night. Several days before the Equinox, the day/night division is almost exact.

Even though day and night won’t be perfectly balanced on March 20th, there is something else that’s neatly aligned: On Thursday, the Sun will rise exactly in the east, and set exactly in the west.

All of the cosmic balancing acts that that seem to go on around the Spring Equinox probably led to the idea that an egg would balance that day, too. But the Earth’s gravity doesn’t shift on the Equinox, and if you can balance an egg on March 20^th, you can do it any day. With a little effort and a level surface, eggs can sit wobble-free on either end. A rough eggshell can make balancing easier.

For more on Equinoxes and eggs, including pictures of eggs balanced on random days of the year, visit www.astrosociety.org/education/publications/tnl/62/equinox.html.

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