Ekuinoks: Pabidaan ralatan
Ma-ulah tungkaran nang isinya ''thumb|The Sun at the moment of the [[March equinox in 2019, when the center of the solar disk contacts the ce...'' |
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Ralatan matan 19 April 2021 11.45
An equinox is the instant of time when the plane of Earth's equator passes through the geometric center of the Sun's disk.[1][2] This occurs twice each year, around 20 March and 23 September. In other words, it is the moment at which the center of the visible Sun is directly above the equator.
The word is derived from the Latin aequinoctium, from aequus (equal) and nox (genitive noctis) (night). On the day of an equinox, daytime and nighttime are of approximately equal duration all over the planet. They are not exactly equal, however, due to the angular size of the Sun, atmospheric refraction, and the rapidly changing duration of the length of day that occurs at most latitudes around the equinoxes. Long before conceiving this equality, primitive equatorial cultures noted the day when the Sun rises due east and sets due west, and indeed this happens on the day closest to the astronomically defined event. As a consequence, according to a properly constructed and aligned sundial, the daytime duration is 12 hours.
In the Northern Hemisphere, the March equinox is called the vernal or spring equinox while the September equinox is called the autumnal or fall equinox. In the Southern Hemisphere, the reverse is true. The dates slightly vary due to leap years and other factors.
Hemisphere-neutral names are northward equinox for the March equinox, indicating that at that moment the solar declination is crossing the celestial equator in a northward direction, and southward equinox for the September equinox, indicating that at that moment the solar declination is crossing the celestial equator in a southward direction.
Since the Moon (and to a lesser extent the planets) causes Earth's orbit to slightly vary from a perfect ellipse, the equinox is officially defined by the Sun's more regular ecliptic longitude rather than by its declination. The instants of the equinoxes are currently defined to be when the apparent geocentric longitude of the Sun is 0° and 180°.[3]
- ^ "Equinoxes". Astronomical Information Center. United States Naval Observatory. 14 June 2019. Diarsipkan dari versi asli tanggal 25 May 2019. Diakses tanggal 9 July 2019.
On the day of an equinox, the geometric center of the Sun's disk crosses the equator, and this point is above the horizon for 12 hours everywhere on the Earth. However, the Sun is not simply a geometric point. Sunrise is defined as the instant when the leading edge of the Sun's disk becomes visible on the horizon, whereas sunset is the instant when the trailing edge of the disk disappears below the horizon. These are the moments of first and last direct sunlight. At these times the center of the disk is below the horizon. Furthermore, atmospheric refraction causes the Sun's disk to appear higher in the sky than it would if the Earth had no atmosphere. Thus, in the morning the upper edge of the disk is visible for several minutes before the geometric edge of the disk reaches the horizon. Similarly, in the evening the upper edge of the disk disappears several minutes after the geometric disk has passed below the horizon. The times of sunrise and sunset in almanacs are calculated for the normal atmospheric refraction of 34 minutes of arc and a semidiameter of 16 minutes of arc for the disk. Therefore, at the tabulated time the geometric center of the Sun is actually 50 minutes of arc below a regular and unobstructed horizon for an observer on the surface of the Earth in a level region
- ^ "ESRL Global Monitoring Division - Global Radiation Group". NOAA. www.esrl.noaa.gov (dalam bahasa Inggris). U.S. Department of Commerce. Diakses tanggal 9 July 2019.
- ^ Astronomical Almanac. United States Naval Observatory. 2008. Glossary.