Wednesday, March 19, 2008

Equinox

On March 20, at 5:28 AM (UT), the Sun crosses our celestial equator during the Vernal Equinox, the mid-point between the longest and shortest days of the year. This is when Spring officially begins.

Predicting the equinoxes and solstices was a critical first step in developing a calendar, planning for the changing seasons, and even for measuring time itself. We know that January is cold and July hot (in the northern hemisphere) because of the patient observations of ancient astronomers who devised a system for measuring time.

Many people think that the seasons changing has something to do with how close the Earth is to the Sun. In fact, this has nothing to do with it. While it is true that the Earth's orbit is elliptical, the eccentricity is very slight and does not seem to have any effect at all on the planet's temperature.
Earth's spinning axis is tipped over by 23.5 degrees relative to the plane in which it orbits. When the northern hemisphere is pointed toward the Sun, we experience Summer. The Sun spends more time above the horizon and rises higher in the sky, which results in more heat per day. Obviously, the reverse is true for the southern hemisphere.

The longest and shortest days of the year, which start the seasons of summer and winter, are called the solstices. The midpoints between them are the equinoxes.

The Spring (or Vernal) Equinox often marks important religious festivals for many cultures worldwide. Easter Sunday is always the Sunday after the first full moon after the Vernal Equinox.
Around the equinoxes, Earth receives equal amounts of light and darkness, resulting in the temperate seasons of spring and autumn.
There is a superstition that it's only possible to balance an egg on its end during the equinox. It's actually possible any day of the year, you just need to keep trying and be patient.

No one knows how or when man developed a precise system for measuring time. Measuring the maximums and minimums for solar position probably led to the first and oldest method for marking time.

This system is somewhat confused by the fact that the equinoxes change slightly each year, because Earth wobbles while it rotates as a spinning top might. Logically, it seems it should take the same time between equinoxes each year, but it doesn't.
Somewhere along the way people noticed that the solstices were moving. The only way this could have been discovered is if records had been kept for thousands of years and comparisons made.

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