Tuesday, March 20, 2012

It's the First Day of Spring!

It’s the first day of spring. For me, I woke up to heavy fog. But fog or not, spring was here. To tell the truth, it had been here for a couple of weeks as flowers bloomed and plants popped out of the ground. The temperature been warm too.

While most of us think of it in terms of flowers, grass needing mowing, birds chirping in the early morning, and warmer temperature, there’s more to it than that. It’s also about the equinox.

The word equinox is derived from the Latin words meaning “equal night.” There are two equinoxes every year – in March and September – when the sun shines directly on the equator and the length of day and night is nearly equal. Seasons are opposite on either side of the equator, so the equinox in March is also known as the "spring equinox" in the northern hemisphere. However, in the southern hemisphere, it's known as the "autumnal (fall) equinox". The Southern Hemisphere: (Australia, New Zealand, South America, Southern Africa). The Northern Hemisphere: (USA, Central America, Canada, Europe, Asia, northern Africa).

The spring and fall equinoxes are the only dates with equal daylight and dark as the Sun crosses the celestial equator—12 hours – all over the world. At the equinoxes, the tilt of Earth relative to the Sun is zero, which means that Earth’s axis neither points toward nor away from the Sun.

But, though accepted, it isn't entirely true. In reality equinoxes don't have exactly 12 hours of daylight. On these two days, the geometric center of the sun is above the horizon for 12 hours, and one might think this would indicate that the length of the daylight would be the same. Sunrise is defined as the instant when the upper edge of the sun's disk becomes visible above the horizon – not when the center of the sun is visible. In the same sense, sunset refers to the moment the upper edge disappears below the horizon. At both instances, the center of the sun is below the horizon, and therefore the equinox day lasts a little longer than 12 hours. The Sun is visible longer than 12 hours on an equinox because the Earth's atmosphere refracts sunlight. Refraction causes the Sun’s upper edge to be visible from Earth several minutes before the edge actually reaches the horizon. In the evening, we can see the sun for several minutes after it has actually dipped under the horizon. This causes every day on Earth, and not just the equinoxes, to appear at least 6 minutes longer than it actually is.

The March equinox occurs the moment the sun crosses the celestial equator. It’s an imaginary line in the sky above the Earth’s equator – from south to north. This happens either on March 19, 20, or 21 every year. On any other day of the year, the Earth's axis tilts a little away from or towards the sun. But on the two equinoxes, the Earth's axis neither tilts away from nor towards the sun.

Equinoxes – along with solstices – have been celebrated in cultures all over the world for as long as we have written history. One of the most famous ancient Spring equinox celebrations was the Mayan sacrificial ritual by the main pyramid in Chichen Itza, Mexico.

In the northern hemisphere, the March equinox marks the start of spring and has long been celebrated as a time of rebirth. Many cultures and religions celebrate or observe holidays and festivals around the March equinox, like the Easter and Passover.

What does spring mean to you. And how do you celebrate it?


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