- September 22 -

Autumnal 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. Seasons are opposite on either side of the equator, so the equinox in September is also known as the 'autumnal (fall) equinox' in the...

Autumnal 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. 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. However, in the southern hemisphere, it's known as the 'spring (vernal) equinox'.

An equinox occurs twice a year (around 20 March and 22 September), when the tilt of the Earth's axis is inclined neither away from nor towards the Sun, the center of the Sun being in the same plane as the Earth's equator. The term equinox can also be used in a broader sense, meaning the date when such a passage happens. The name 'equinox' is derived from the Latin aequus (equal) and nox (night), because around the equinox, night and day are about equal length.

At an equinox the Sun is at one of two opposite points on the celestial sphere where the celestial equator (i.e. declination 0) and ecliptic intersect. These points of intersection are called equinoctial points: classically, the vernal point (RA = 00h 00m 00s and longitude = 0o) and the autumnal point (RA = 12h 00m 00s and longitude = 180o). By extension, the term equinox may denote an equinoctial point.
The equinoxes are the only times when the subsolar point (the place on the Earth's surface where the center of the Sun is exactly overhead) is on the Equator. The subsolar point crosses the Equator moving northward at the March equinox and moving southward at the September equinox. (Since the sun's ecliptic latitude isn't exactly zero it isn't exactly above the equator at the moment of the equinox, but the two events usually occur less than 30 seconds apart.)
The equinoxes are the only times when the terminator is inclined 90° to the Earth's Equator (while at solstices, that inclination reaches its minimum of 66.5°, corresponding to 90° minus Earth's axial tilt).
Source:
http://www.timeanddate.com/calendar/september-equinox.html
http://en.wikipedia.org/wiki/Equinox

Categories:

You may also like...

Interesting events from Nature category.

World Wetlands Day

World Wetlands Day is celebrated internationally each year on 2 February. It marks the anniversary of the signing of the Convention on Wetlands of International Importance (Ramsar Convention) in Ramsar, Iran, on 2 February 1971. World Wetlands Day was first celebrated in 1997. Since then...

International Mother Earth Day

Mother Earth is a common expression for the planet Earth in a number of countries and regions, which reflects the interdependence that exists among human beings, other living species and the planet. For instance, Bolivians call Mother Earth Pachamama and Nicaraguans refer to her as...

International Plastic Bag Free Day

International plastic bag free day is a global initiative that aims to eliminate or radically reduce the use of single-use plastic bags in the world and to make society aware of the over-consumption of plastic bags and disposable products and work towards developing more responsible and more...

Simplicity Day

July 12 is Simplicity Day. Voluntary simplicity is a not a new concept. In fact it speaks to ancient wisdom that our societies seem to have lost in modern times. Choosing to live a life of simplicity is not about choosing to live in poverty; it is about living in balance. Based on three key...

Butterfly and Hummingbird Day

Nothing brings beauty to your yard or garden like butterflies and hummingbirds. Their small size only accentuates their beauty, packing more awesomeness per square inch than even the most gorgeous of sunsets. But their beauty is more than just skin deep, because they also pollinate flowers and...

International Day for Preventing the Exploitation of the Environment in War and Armed Conflict

The United Nations’ (UN) International Day for Preventing the Exploitation of the Environment in War and Armed Conflict is annually held on November 6. It aims to educate people about the damaging effects of war and armed conflict on the environment. History On 5 November...

Download KeepIn Calendar

Enjoy the interesting stories even on your phone or tablet