Gerhana pusat

Istilah bagi gerhana pusat

    Gerhana Pusat "Central eclipse" sering kali digunakan sebagai istilah umum bagi gerhana penuh, gerhana anular, atau gerhana kacukan. Ini bagaimanapun, tidak tepat: istilah bagi gerhana pusat merupakan di mana garis tengah umbra menyentuh permukaan bumi. Ia adalah mungkin, sungguhpun amat jarang, sebahagian umbra melintasi Bumi (dengan itu menghasilkan gerhana anular atau gerhana penuh), tetapi tidak garis tengahnya. Ini sering kali dikenali sebagai gerhana penuh tidak tengah atau gerhana anular. The next non-central solar eclipse will be on April 29, 2014. This will be an annular eclipse. The next non-central total solar eclipse will be on April 9, 2043.
Fasa yang dilihat semasa gerhana penuh dikenali sebagai:
  • First Contact — when the moon's shadow first becomes visible on the solar disk. Some also name individual phases between First and Second Contact e.g. Pac-Man phase.
  • Second Contact — starting with Baily's Beads {caused by light shining through valleys on the moon's surface} and the Diamond Ring. Almost the entire disk is covered.
  • Totality — with the shadow of the moon obscuring the entire disk of the sun and only the corona visible
  • Third Contact — when the first bright light becomes visible and the shadow is moving away from the sun. Again a Diamond Ring may be observed
Rajah gerhana suria (tidak menurut skala)

     Rajah di atas menunjukkan susunan Matahari, Bulan, dan Bumi semasa gerhana matahari. Kawasan kelabu gelap di bawah Bulan merupakan umbra, di mana Matahari tersembunyi sepenuhnya oleh Bulan. Kawasan kecil di mana bayangan gelap umbra menyentuh permukaan Bumi merupakan di mana kawasan gerhana penuh boleh dilihat. Kawasan kelabu cerah yang lebih besar merupakan penumbra, di mana hanya gerhana separuh boleh dilihat.

     The Moon's orbit around the Earth is inclined at an angle of just over 5 degrees to the plane of the Earth's orbit around the Sun (the ecliptic). Because of this, at the time of a new moon, the Moon will usually pass above or below the Sun. A solar eclipse can occur only when the new moon occurs close to one of the points (known as nodes) where the Moon's orbit crosses the ecliptic.

     As noted above, the Moon's orbit is also elliptical. The Moon's distance from the Earth can vary by about 6% from its average value. Therefore, the Moon's apparent size varies with its distance from the Earth, and it is this effect that leads to the difference between total and annular eclipses. The distance of the Earth from the Sun also varies during the year, but this is a smaller effect. On average, the Moon appears to be slightly smaller than the Sun, so the majority (about 60%) of central eclipses are annular. It is only when the Moon is closer to the Earth than average (near its perigee) that a total eclipse occurs.

     The Moon orbits the Earth in approximately 27.3 days, relative to a fixed frame of reference. This is known as the sidereal month. However, during one sidereal month, the Earth has revolved part way around the Sun, making the average time between one new moon and the next longer than the sidereal month: it is approximately 29.5 days. This is known as the synodic month, and corresponds to what is commonly called the lunar month.
A Total eclipse in the umbra.
B Annular eclipse in the antumbra.
C Partial eclipse in the penumbra

     The Moon crosses from south to north of the ecliptic at its ascending node, and vice versa at its descending node. However, the nodes of the Moon's orbit are gradually moving in a retrograde motion, due to the action of the Sun's gravity on the Moon's motion, and they make a complete circuit every 18.6 years. This means that the time between each passage of the Moon through the ascending node is slightly shorter than the sidereal month. This period is called the draconic month.

     Finally, the Moon's perigee is moving forwards in its orbit, and makes a complete circuit in about 9 years. The time between one perigee and the next is known as the anomalistic month.
The Moon's orbit intersects with the ecliptic at the two nodes that are 180 degrees apart. Therefore, the new moon occurs close to the nodes at two periods of the year approximately six months apart, and there will always be at least one solar eclipse during these periods. Sometimes the new moon occurs close enough to a node during two consecutive months. This means that in any given year, there will always be at least two solar eclipses, and there can be as many as five. However, some are visible only as partial eclipses, because the umbra passes above Earth's north or south pole, and others are central only in remote regions of the Arctic or Antarctic.


     During a central eclipse, the Moon's umbra (or antumbra, in the case of an annular eclipse) moves rapidly from west to east across the Earth. The Earth is also rotating from west to east, but the umbra always moves faster than any given point on the Earth's surface, so it almost always appears to move in a roughly west-east direction across a map of the Earth (there are some rare exceptions to this which can occur during an eclipse of the midnight sun in Arctic or Antarctic regions).

     The width of the track of a central eclipse varies according to the relative apparent diameters of the Sun and Moon. In the most favourable circumstances, when a total eclipse occurs very close to perigee, the track can be over 250 km wide and the duration of totality may be over 7 minutes. Outside of the central track, a partial eclipse can usually be seen over a much larger area of the Earth.

Occurrence and cycles
Total Solar Eclipse Paths: 1001–2000. This image was merged from 50 separate images from NASA.

     Total solar eclipses are rare events. Although they occur somewhere on Earth every 18 months on average, it has been estimated that they recur at any given place only once every 370 years, on average. The total eclipse only lasts for a few minutes at that location, as the Moon's umbra moves eastward at over 1700 km/h. Totality can never last more than 7 min 31 s, and is usually much shorter: during each millennium there are typically fewer than 10 total solar eclipses exceeding 7 minutes. The last time this happened was June 30, 1973 (7 min 3 sec). Observers aboard a Concorde aircraft were able to stretch totality to about 74 minutes by flying along the path of the Moon's umbra. The next eclipse exceeding seven minutes in duration will not occur until June 25, 2150. The longest total solar eclipse during the 8,000-year period from 3000 BCE to 5000 AD will occur on July 16, 2186, when totality will last 7 min 29 s. For comparison, the longest eclipse of the 21st century occurred on July 22, 2009 and lasted 6 min 39 sec.

     If the date and time of any solar eclipse are known, it is possible to predict other eclipses using eclipse cycles. Two such cycles are the Saros and the Inex. The Saros cycle is probably the best known, and one of the most accurate, eclipse cycles. The Inex cycle is itself a poor cycle, but it is very convenient in the classification of eclipse cycles. After a Saros cycle finishes, a new Saros cycle begins one Inex later, hence its name: in-ex. A Saros cycle lasts 6,585.3 days (a little over 18 years), which means that after this period a practically identical eclipse will occur. The most notable difference will be a shift of 120° in longitude (due to the 0.3 days) and a little in latitude. A Saros series always starts with a partial eclipse near one of Earth's polar regions, then shifts over the globe through a series of annular or total eclipses, and ends at the opposite polar region. A Saros (series) lasts 1226 to 1550 years and 69 to 87 eclipses, with about 40 to 60 central.

Perkalaan setahun

     Gerhana matahari boleh berlaku 2 hingga 5 kali dalam satu tahun takwim. Sejak kalander Gregorian digunakan pada tahun 1582, tahun yang mempunyai lima kali gerhana matahari berlaku pada tahun 1693, 1758, 1805, 1823, 1870, dan 1935. Ulangan berikutnya akan berlaku pada 2206.

Sebagai contoh 5 gerhana matahari pada tahun 1935 adalah:

January 5
February 3

June 30
July 30

December 25

Saros 111

Saros 149

Saros 116

Saros 154

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