When the equinox gene appears, repair transitions into regrowth

When animals experience a large injury, such as the loss of a limb, the body immediately begins a wound healing response that includes sealing the wound site and repairing local damage. In many animals, including humans, ...

Why day and night aren't equal length on an equinox

Winter in the northern hemisphere and summer in the southern hemisphere are both coming to an end. That means the days and nights are becoming roughly equal in length, and the path the sun traces across the sky is changing.

Image: Saturn at equinox

Saturn is famous for its bright, glorious rings but in this picture, taken during Saturn's 2009 equinox, the rings are cast in a different light as sunlight hits the rings edge-on.

Winter solstice: The astronomy of Christmas

From the Neolithic to present times, the amount of sunlight we see in a day has had a profound impact on human culture. We are fast approaching the winter solstice for the Northern hemisphere, which takes place on December ...

Saturn's little wavemaking moon

Captured on January 15, this narrow-angle Cassini image shows an outer portion of Saturn's A ring on the left and the ropy F ring crossing on the right. The thin black line near the A ring's bright edge is the Keeler Gap, ...

SDO enters its semiannual eclipse season

(Phys.org)—Twice a year, for three weeks near the equinox, NASA's Solar Dynamics Observatory (SDO) moves into its eclipse season—a time when Earth blocks its view of the sun for a period of time each day.

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Equinox

An equinox occurs twice a year, 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, the night and day have approximately 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 and the autumnal point. By extension, the term equinox may denote an equinoctial point.

An equinox happens each year at two specific moments in time (rather than two whole days), when there is a location (the subsolar point) on the Earth's equator, where the center of the Sun can be observed to be vertically overhead, occurring around March 20/21 and September 22/23 each year.

Although the word equinox is often understood to mean "equal [day and] night," this is not strictly true. For most locations on earth, there are two distinct identifiable days per year when the length of day and night are closest to being equal; those days are referred to as the "equiluxes" to distinguish them from the equinoxes. Equinoxes are points in time, but equiluxes are days. By convention, equiluxes are the days where sunrise and sunset are closest to being exactly 12 hours apart.

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