How does a solar eclipse occur?

Fig 2.1A solar eclipse (eclipse of the Sun) occurs when the Moon, in its orbit around the Earth, passes between the Earth and the Sun and casts a shadow on the Earth. There are two parts to the Moon's shadow. The dark inner cone shaped part is called the umbra and the lighter outer part is called the penumbra. From within the umbra the Sun is completely hidden and for an observer on the Earth in the umbra there will be a total solar eclipse as the Moon will completely obscure (eclipse) the Sun. From within the penumbra the Moon will only partially obscure the Sun so that from these locations there will be a partial solar eclipse. See figure 2-1.

The period during which an eclipse is total at any location is called totality. As the Moon orbits the Earth and the Earth rotates, the Moon's shadow moves across the Earth's surface from west to east with the umbra tracing out a narrow path called the path of totality. For any location in the path of totality, the Moon, over a period of an hour or so, will firstly appear to progressively cover the Sun in the partial phase of the eclipse. As the umbra arrives at that location, the Sun will be completely covered and there will be a total eclipse. As the umbra moves on, the Moon from that location will appear to gradually uncover the Sun in the final partial phase of the eclipse, which generally takes about another hour.

At any point on the Earth's surface outside the path of totality but crossed by the penumbra, the Moon will only partly cover the Sun, reaching a point of maximum coverage and then move on to uncover the Sun. So this location will experience only a partial eclipse. Any point on the Earth's surface not crossed by the penumbra will not see any eclipse. Sometimes the umbra does not cross the Earth (passing above or below the Earth in space) and there may be a partial eclipse without there being a total eclipse anywhere.

The shadow of the umbra will travel across the Earth in a path thousands of kilometres long, usually taking about three hours or so and travelling at speeds up to several thousands of kilometres per hour.

The Sun and Moon appear to be roughly the same size in the sky but their sizes are actually very different. By a fortuitous coincidence the Sun is about 400 times larger than the Moon but the Sun is also about 400 times further away. This can be confirmed by using the figures for diameter and the average distances for the Sun and Moon given in Sun facts and Statistics and Moon Facts and Statistics. This close match between the apparent size of the Sun and the Moon is a key reason for some of the spectacular effects of a total solar eclipse and is the reason why the width of the path of totality is quite narrow, typically no more than about 270 km. The maximum duration of the total part of any eclipse depends mainly on the actual distances of the Sun and Moon from the Earth at the time and can last from only a few seconds to over seven minutes.

 

Annular solar eclipse

Fig 2.2a

As the Moon is in an elliptical orbit around the Earth, its distance from the Earth varies and as a consequence its apparent size in the sky varies. Likewise, the apparent size of the Sun varies because the Earth moves in an elliptical orbit around the Sun. These two factors combine to give the relative sizes of the Sun and the Moon. Sometimes the Moon appears larger than the Sun and at that time if the Moon passes in front of the Sun it is easily able to completely cover the Sun and a total solar eclipse can occur.

At other times the Moon will appear smaller than the Sun and at these times if the Moon passes in front of the Sun it cannot completely cover the Sun and a ring or annulus of the bright surface of the Sun remains. This is called an annular (Latin meaning ring) eclipse. The sky does not go very dark and none of the spectacular events of a total solar eclipse can be seen. The umbra of the Moon's shadow does not reach the Earth as figure 2-2 shows. The part of the Moon's shadow that extends beyond the end of the umbra is called the antumbra. Observers within the antumbra will experience an annular eclipse.

See the sections on annual solar eclipses and the annular eclipse of 2013 for more information.

Other types of eclipses

The paragraphs above describe partial, total and annular eclipses. Sometimes when there is a close match between the apparent sizes of the Sun and the Moon, the Moon's umbra will be just long enough to touch the Earth in the central part of the path but not long enough to reach the Earth at one or both ends of the path. In this case the eclipse path will normally start as annular, change to total then change back to annular. In this case it is called a Hybrid eclipse. It can also be called an annular – total eclipse.

A central eclipse is a solar eclipse in which the central axis of the Moon's shadow traverses the Earth. Central solar eclipses can be total, annular or hybrid.

Frequency of eclipses

Solar eclipses are not especially rare. Partial eclipses, in which the umbra of the Moon’s shadow never touches the Earth, occur about three times every year. On average, there is a total solar eclipse somewhere on the Earth about every 18 months. Annular eclipses occur a little more frequently than total solar eclipses.

The plane of the Earth’s orbit around the Sun is called the ecliptic. The Moon’s orbit around the Earth is tilted at about 5 degrees with respect to the ecliptic. So at each new moon, the Moon usually tracks “above” or “below” the Sun and is not able to block the Sun. It is only when the new moon is near to one of the two locations where the Moon’s orbit crosses the plane of the Earth’s orbit that an eclipse is possible. These crossing points are called nodes. There is an eclipse season about every 6 months when the Sun is close to one of the nodes and an eclipse is possible. In 2012 this occurs in May and November. But the time between each successive crossing of the node and back again is less than a year (this period is called an eclipse year and is 346.6 days). So over time the nodes move and the eclipse season comes earlier. This is called regression of the nodes.

Solar eclipses repeat in a regular pattern called a Saros which is a period of 18 years 11⅓ days. This is because 223 cycles from one full moon to the next (called a lunar month) and 19 eclipse years both equal this period. In addition 239 cycles of the Moon moving in its elliptical orbit from apogee to perigee and back again (called the anomalistic month) also is very close to this period. So a very similar eclipse will occur 18 years and 11 days after a previous one. Because of the ⅓ of a day, it occurs ⅓ of the way round the world. All eclipses belong to a saros series and each has been given a number. The eclipse of 14 November 2012 is part of saros number 133.

Fig 2.3

Eclipse timing and eclipse contacts

Eclipse timing is usually given in terms of eclipse contacts. Figure 2-3 shows the four eclipse contacts for a total solar eclipse. The Moon progressively covers the Sun until the brief total eclipse and then it gradually uncovers the Sun. As the Sun moves across the sky, the Moon appears to move at a slower rate causing it to move across in front of the Sun.  Contacts are the points at which the apparent edges of the Sun and Moon cross one another. 

First Contact (C1): the Moon first begins to cover the Sun and is the start of the partial phase.

Second Contact (C2): the Sun is first completely covered and is the start of the total part of the eclipse.

Third Contact (C3): the Sun is at the point of being uncovered and is the end of the total part of the eclipse.

Fourth Contact (C4) is when the Moon lastly touches the Sun and is the end of the final partial phase.

From a location where there is only a partial solar eclipse, there is only first contact and fourth contact.

Lunar eclipses - partial and total

Fig 2.4b

A lunar eclipse occurs when the shadow of the Earth falls on the Moon as shown in Figure 2-4. As the Earth orbits the Sun, the shadow of the Earth moves through space on the opposite side to the Sun. The plane of the Moon's orbit is tilted at about 5° with respect to the ecliptic. Thus, as the Moon orbits the Earth each month, at each full moon it usually passes above or below the Earth's shadow. The Earth's shadow is a cone that, at the Moon's distance from the Earth, is more than twice as broad as the Moon's diameter. But every few months, during the eclipse season as described above, the Moon passes either partially or entirely into the Earth's shadow.

The Earth's shadow has a darker inner cone, the umbra, from which the Sun cannot be seen at all, and an outer cone, the penumbra, from which part of the Sun can be seen. Most of the penumbra is not dark enough to have a noticeable effect on the Moon's brightness, so penumbral eclipses are largely ignored.

During a partial lunar eclipse, the umbra of the Earth's shadow advances onto part of the Moon's surface and then moves off the other side. No other effect is seen. 

Fig 2.5

During a total lunar eclipse, the umbra advances over the Moon and then gradually covers the Moon until it is immersed in the Earth's shadow. Though no sunlight falls directly on the totally eclipsed Moon, some sunlight is bent round through the Earth's atmosphere thereby illuminating the Moon. From this refracted sunlight, the blue light is removed when the light is scattered in the Earth's atmosphere. This creates blue skies overhead for people on Earth; only the red light gets through to the Moon. Thus the totally eclipsed Moon appears reddish as shown in Figure 2-5. Just how reddish it looks depends on whether volcanic dust is present in the Earth's atmosphere; the dust makes the Moon look darker and less reddish. Even giant storms or cloudy regions on Earth can affect the Earth's shadow, perhaps making the darkness of the shadow appear uneven on the Moon. The Moon will also appear less evenly illuminated if it passes closer to the sides of the umbra instead of through the umbra's centre.

Following are dates of upcoming lunar eclipses visible from Australia:

  • 26 April 2013 Partial Lunar Eclipse (morning)
  • 15 April 2014 Total Lunar Eclipse (evening)
  • 08 October 2014 Total Lunar Eclipse (evening)

Eclipse Websites

WARNING.  Never look directly at the bright surface of the Sun without suitable eye protection as permanent eye damage may result. This applies at any time and especially during the partial phases of a solar eclipse. Refer to “How to observe the Sun safely”.

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