Equinox

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For other uses, see Equinox (disambiguation).

Template:Solstice-equinox An equinox in astronomy is the moment when the Sun passes over the equator. The event occurs twice a year, around March 21 and September 23. The word equinox derives from the Latin word for equal night. The equinoxes are the two days each year when the middle of the Sun is an equal amount of time above and below the horizon for every location on Earth.

In the Northern Hemisphere, the March equinox is known as the vernal equinox and the September equinox is the autumnal equinox. In the Southern Hemisphere, the names are reversed.

In practice, at the equinox, the day is longer than the night. Commonly the day is defined as the period that sunlight may reach the ground in absence of local obstacles. This is firstly because the Sun does not appear as a single point of light, but as a disc. So when the middle of the Sun is still below the horizon, the topmost edge is already visible and sheds light. Furthermore, the atmosphere refracts light downwards, so even when the topmost edge of the Sun is really still below the horizon, its rays already reach around the horizon to the ground. These effects together make the day about 14 minutes longer than the night (at the equator, and more towards the poles). The real equality of day and night happens a few days towards the winter side of the equinox.

One of the effects of equinoctial periods is their temporary disruptive effect on Communications satellites. For most geostationary satellites, there is almost always a point when the sun comes directly behind the satellite relative to Earth. The Sun's immense power and broad radiation spectrum overload the Earth station reception circuits with noise and, depending on antenna size and other factors, temporarily disrupt or degrade the circuit. The duration of those effects varies but can range from an hour to a few minutes.

1 Equinoctial point
2 Apparent behaviour of the Sun
- 2.1 March equinox
- 2.2 September equinox
3 See also
4 External links

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Equinoctial point

The equinoxes can also be interpreted as virtual points in the sky. Although, during full daylight, stars other than the Sun are overwhelmed by sunlight, making it hard to see where the Sun is compared to other celestial bodies, the Sun does have a position (as seen from Earth) relative to the other stars. As 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 Earth's orbit projected against the whole sky. The other bright planets like Venus, Mars and Saturn, also appear to move along the ecliptic, because their orbits are in a similar plane to Earth's.

Another virtual circle in the sky is the celestial equator, or the projection of the plane of Earth's equator against the whole sky. Because 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 by about 23.5 degrees.

Twice a year, the Sun, making its progress around the ecliptic, crosses the plane of Earth's equator. The two intersections between ecliptic and celestial equator are the equinoctial points.

The time at which the Sun passes through each equinoctial point can be calculated precisely and the equinox actually occurs at a particular moment. The days at which the Sun passes through these points may occur at different dates at different places on the Earth because of the timezone difference (occurring before midnight in one place and after midnight in the other).

Scientist Hipparchus used the equinoctial point to divide the day into twenty-four equinoctial hours (the length required for the Earth to perform one full rotation). Before the method was discovered, the hours of a day varied in length and according to the season; it was longer in the summer and shorter in the winter.

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Apparent behaviour of the Sun

Image:Earth-lighting-equinox EN.png On the equinoxes, everywhere over the globe, the Sun rises true east (parallel to lines of latitude), sets at true west, and the length of the day equals the length of the night.

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March equinox

At the North pole the Sun passes from a 6-month-long night to a 6-month-long day.

At the Arctic circle the Sun reaches an altitude of 23.5° in the South.

At the Tropic of Cancer the Sun reaches an altitude of 66.5° in the South.

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 Capricorn the Sun reaches an altitude of 66.5° in the North.

At the Antarctic circle the Sun reaches an altitude of 23.5° in the North.

At the South pole the Sun passes from a 6-month-long day to a 6-month-long night.

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September equinox

At the North pole the Sun passes from a 6-month-long day to a 6-month-long night.

At the Arctic circle the Sun reaches an altitude of 23.5° in the South.

At the Tropic of Cancer the Sun reaches an altitude of 66.5° in the South.

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 Capricorn the Sun reaches an altitude of 66.5° in the North.

At the Antarctic circle the Sun reaches an altitude of 23.5° in the North.

At the South pole the Sun passes from a 6-month-long night to a 6-month-long day.

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