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.

11/11/11: Anthropologist debunks doomsday myths

University of Kansas anthropologist and Maya scholar John Hoopes and his students are watching predicted doomsday dates such as 11/11/11 and Dec. 21, 2012, with considerable skepticism.

Spring on Titan brings sunshine and patchy cloud

Titan's northern hemisphere is set for mainly fine spring weather, with polar skies clearing since the equinox in August last year. Cassini’s VIMS instrument has been monitoring clouds on Titan continuously since the spacecraft ...

Super harvest moon to produce rare twilight glow

For the first time in almost 20 years, northern autumn is beginning on the night of a full Moon. The coincidence sets the stage for a "Super Harvest Moon" and a must-see sky show to mark the change of seasons.

Cassini Finishes Saturnian Doubleheader

(PhysOrg.com) -- NASA's Cassini spacecraft completed its double flyby this week, swinging by Saturn's moons Titan and Dione with no maneuver in between. The spacecraft has beamed back stunning raw images of fractured terrain ...

Cassini Shows Saturnian Roller Derby, Strange Weather

(PhysOrg.com) -- The seemingly serene orb of Saturn is in fact a gas giant with extraordinary patterns of charged particles and rough and tumble roller derbies for rings. Such are the findings of NASA's Cassini spacecraft ...

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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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