Updated November 10, 2013

Eclipses

Eclipse and lunar phases

An eclipse requires an alignment of three celestial bodies, three bodies are the Sun, Earth and Moon. Lunar eclipses are occultations of the Moon by the Earth's shadow while solar eclipses are eclipses of the sun in the shadow of the Moon. These celestial concordances are frequently observed, at least one of each (lunar eclipse and solar eclipse) every 6 months. The Moon appears to us with phases, i.e. in different lights of the Lunar sphere. This is because the Moon revolves around the Earth and the relative positions of the Sun, Earth and Moon in constant motion. As the moon only reflects light from the Sun, the only visible part of the Moon is one that is geared to both the Earth and to the Sun. The periodicity of phases of the moon is 29.5 days, or a lunation or lunar month although there are several lunar month. Eclipses take place at syzygies, i.e. when there is a quasi alignment between the Sun, Earth and Moon. The eclipse occurs at new moon, the lunar eclipse occurs at full Moon. A happy coincidence makes the sun eclipse, spectacular.

The ratio between the diameter of the Sun (1,400,000 km) and the diameter of the Moon (3,500 km) is ≈ 1/400, the ratio between the distance from the Sun (149,600,000 km) and the distance of the Moon (384,000 km) is ≈ 1/400. As the Sun is 400 times larger and 400 times farther than the moon, their apparent size in the sky are almost identical (≈ 0.5 ° or ≈ 30 minutes of angle). In fact, the apparent size of the Moon varies between 29.3' and 33.5' and the apparent size of the Sun varies between 31.5' and 32.5'. This temporal coincidence is the basis of mythological interpretations of certain civilizations since the moon sometimes completely mask the Sun. Today, a total solar eclipse arouses passion crowds. Some enthusiasts are called "eclipse chasers", they stalk eclipses around the world. However, this coincidence will disappear in hundreds of millions of years there will be only annular eclipses because the Moon is moving away from Earth 4 cm per year. Today solar eclipses are annular or total or mixed .

Image: The phases of the Moon succeed in a cycle of 29.5 days, called "lunar month." 8 phases of the moon are: full moon, the first crossing of moon, first quarter, waxing gibbous, full moon, waning gibbous, last quarter and waning crescent moon. The eclipse occurs at new moon, the lunar eclipse occurs at full moon.
Credit image: astronoo.com

Periods of revolution of the Moon

The sidereal period of revolution of the Moon is measured relative to the stars. This is the period after which we will find the Moon in the same position as seen from the Earth compared to a star. The sidereal period of revolution is 27.321582 days.
The synodic period of revolution, this time measured relative to the Sun, is 29.530589 days. This explains why solar eclipses and lunar eclipses successive 15 days. If the sidereal period of revolution of the Moon is shorter than the synodic period of revolution, because for a month Earth advanced in its orbit and it will take about 2 days for the Moon to reach the same position the Sun. The phenomenon is the same for the Earth whose sidereal period (relative to the stars) is shorter than the synodic period (relative to the Sun), about 4 minutes, about 23H 56mn or 0.99726968 day. The synodic period is longer 1/365th because during 24H, the Earth advanced in its orbit around the Sun, 1/365th. 1/365th of 24H is about 4 minutes. Although the synodic period of revolution is 29.5 days, the eclipses do not occur every month, the Moon passes above or below the Sun as the plane of the orbit of the Moon around the Earth and the plane of the orbit of the Earth around the Sun (ecliptic) do not overlap, they make an angle of about 5.145 °. If the Moon was moving around the Earth in the same plane as the Earth around the Sun, i.e. the plane of the ecliptic, there would be a lunar eclipse every full moon and a solar eclipse every new moon, all 29.5 days, each separated by 15 days.

In fact, the alignment of three bodies can only happen if the Moon is very close to the plane of the ecliptic, it is for this reason that this plan was so called in relation to the eclipse. The place where the Moon passes above the plane of the ecliptic is called the ascending node, conversely where the Moon passes below the plane of the ecliptic is called the descending node. To attend a total or partial eclipse, it is necessary that the Moon is on the line of nodes is the only place where the alignment of the three asters is perfect. It has between four and six eclipses in the year, at least two of the Sun and at least two of the Moon. In summary eclipses take place at syzygies (alignment of three asters) near the line of nodes. Around the passage of the nodes there is a period of 35 day where the eclipse can occur. In 34.5 days there may be two solar eclipses and two lunar eclipses since the lunation lasts 29.5 days. Eclipses are spaced 1 month to 6 or 5 months. The distance Earth Moon is between 363,104 km and 405,696 km, more the Moon is far from Earth at the time of syzygy and more the eclipse is partial. The interval between two total solar eclipses in the same season (eg summer to another) is usually 12 lunar months is 354.4 days (1 year - 11 days). So we could attend the August 1, 2008, a total eclipse in Mongolia, and on 22 July 2009 at a total eclipse in China and 11 July 2010 a total eclipse in Polynesia.

Image: Map apparent orbits of the Moon and the Sun as viewed from Earth. If the Moon was moving around the Earth in the same plane as the Earth around the Sun, i.e. the plane of the ecliptic, there would be a lunar eclipse every full moon and a solar eclipse every new moon, all 29.5 days, each separated by 15 days. Period eclipses last 34.5 days, so there 2 or 3 eclipses during this period. There are also 1, 5 or 6 months between two eclipses of the sun or moon consecutive. In fact, the alignment of three bodies can only happen if the Moon is very close to the plane of the ecliptic, it is for this reason that this plan was so called in relation to the eclipse. The plane of the orbit of the Moon is animated by a movement of precession, so the nodes traverse the ecliptic in a retrograde direction (clockwise counterclockwise) following a period of 18.61 years. Credit astronoo.com