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The word "equinox" comes from the Latin for "equal night". The equinoxes in March and September are the two times each year when day and night are of equal duration. For terms of measuring the length of day on the equinox, sunrise starts when the sun is half way above the horizon and sunset is when the sun is halfway under the horizon. Using this definition, the length of the day is precisely 12 hours. At the equinoxes, the sun rises due east and sets due west.
In the northern hemisphere, the March equinox is the vernal equinox; the September equinox is the autumnal equinox. In the southern hemisphere, these names are sometimes transposed.
The equinoxes can also be regarded as points in the sky. Although, during full daylight, other stars are overwhelmed by sunlight, making it hard to see where the sun is compared to other celestial bodies, the sun does have a definite position relative to the other stars. As the Earth moves around the sun, the apparent position of the sun relative to the other stars moves in a full circle over the period of a year. This circle is called the ecliptic, and is also the plane of the Earth's orbit projected against the whole sky. The other naked-eye planets like Venus, Mars and Saturn, also appear to move along the ecliptic, because their orbits are in a similar plane to Earth's.
The other circle in the sky is the celestial equator, or the projection of the plane of the Earth's equator against the whole sky. Because the Earth's axis of rotation is tilted relative to the plane of Earth's orbit around the sun, the celestial equator is inclined to the ecliptic. Twice a year, the sun, making its progress around the ecliptic, crosses the plane of the Earth's equator. These two points are the equinoxes. The time at which the sun passes through each equinox point can be calculated precisely—so the equinox is actually a particular moment, rather than a whole day.
On the equinoxes the sun rises exactly at East and sets exactly at West everywhere, and the length of the day equals the length of the night.
At the equator the sun rises in a vertical line from the East on the horizon to the zenith, and then sets in a vertical line from the zenith to the West on the horizon.
At the Tropic of CancerThe Tropic of Cancer is one of the five major circles of latitude that mark maps of the Earth. This is the parallel of latitude that runs 23° 26' 22" north of the Equator, and is the farthest northern latitude at which the sun can appear directly overhead the sun passes from South, where it reaches its maximum altitude of that day, that is 66°33'.
At the Tropic of CapricornThe Tropic of Capricorn is one of the five major circles of latitude that mark maps of the Earth. This is the parallel of latitude that runs 23° 26' 22" south of the Equator, and is the farthest southern latitude at which the sun can appear directly overh the sun passes from North, where it reaches its maximum altitude of that day, that is 66°33'.
At the North pole the sun passes from a 6-month-long night to a 6-month-long day.
At the South pole the sun passes from a 6-month-long day to a 6-month-long night.
At the equator the sun rises in a vertical line from the East on the horizon to the zenith, and then sets in a vertical line from the zenith to the West on the horizon.
At the Tropic of CancerThe Tropic of Cancer is one of the five major circles of latitude that mark maps of the Earth. This is the parallel of latitude that runs 23° 26' 22" north of the Equator, and is the farthest northern latitude at which the sun can appear directly overhead the sun passes from South, where it reaches its maximum altitude of that day, that is 66°33'.
At the Tropic of CapricornThe Tropic of Capricorn is one of the five major circles of latitude that mark maps of the Earth. This is the parallel of latitude that runs 23° 26' 22" south of the Equator, and is the farthest southern latitude at which the sun can appear directly overh the sun passes from North, where it reaches its maximum altitude of that day, that is 66°33'.
At the North pole the sun passes from a 6-month-long day to a 6-month-long night.
At the South pole the sun passes from a 6-month-long night to a 6-month-long day.
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