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There are two equinoxes every year – in March and September –

when the Sun shines directly on the equator and the length of night and day are nearly equal.

 

What is the September Equinox?

There are two equinoxes every year – in September and March – when the sun shines directly on the equator and the length of day and night is nearly equal.

The September equinox (or Southward equinox) is the moment when the sun appears to cross the celestial equator, heading southward. Due to differences between the calendar year and the tropical year, the September equinox can occur at any time from the 21st to the 24th day of September.

At the equinox, the sun rises directly in the east and sets directly in the west. Before the Southward equinox, the sun rises and sets more and more to the north, and afterwards, it rises and sets more and more to the south. In the Northern Hemisphere the September equinox is known as the autumnal equinox. In the Southern Hemisphere it is known as the vernal or spring equinox.

Spring in the South

Seasons are opposite on either side of the equator, so the equinox in September is also known as the Autumnal (fall) equinox in the northern hemisphere. In the southern hemisphere, it's known as the Spring (vernal) equinox.

Sun Crosses Celestial Equator

The September equinox occurs the moment the Sun crosses the celestial equator – the imaginary line in the sky above the Earth’s equator – from north to south. This happens either on September 22, 23, or 24 every year.

10 Facts About the March Equinox

At the equinox, the Sun rises directly in the east and sets directly in the west. However, because of refraction it will usually appear slightly above the horizon at the moment when its "true" middle is rising or setting. For viewers at the north or south poles, it moves virtually horizontally on or above the horizon, not obviously rising or setting apart from the movement in "declination" (and hence altitude) of a little under a half (0.39) degree per day.

For observers in either hemisphere not at the poles, the further one goes in time away from the September equinox in the 3 months before that equinox, the more to the north the Sun has been rising and setting, and for the 3 months afterwards it rises and sets more and more to the south.

The Axial Tilt

The Earth's axis is always tilted at an angle of about 23.5° in relation to the ecliptic, the imaginary plane created by the Earth's path around the Sun. On any other day of the year, either the southern hemisphere or the northern hemisphere tilts a litte towards the Sun. But on the two equinoxes, the tilt of the Earth's axis is perpendicular to the Sun's rays, like the illustration shows.

Why "Equinox"?

On the equinox, night and day are nearly exactly the same length – 12 hours – all over the world. This is the reason it's called an "equinox", derived from Latin, meaning "equal night". However, even if this is widely accepted, it isn't entirely true. In reality equinoxes don't have exactly 12 hours of daylight

Traditions and Folklore

In the northern hemisphere the September equinox marks the start of fall (autumn). Many cultures and religions celebrate or observe holidays and festivals around the September equinox.

 

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