Eclipses

A Solar eclipse takes place when the Moon passes in front of the Sun as seen from somewhere on the Earth.  The distance of the Moon from the Earth varies - the closer the Moon is, the larger it appears to be.  When the Moon is at its closest to the Earth (near perigee) it appears large enough to completely cover the Sun.  If this happens a total eclipse of the Sun can be seen - if you are at the right place on the Earth.

At times when the Moon is more distant from the Earth its apparent size is slightly less than that of the Sun.  In which case if an eclipse occurs there will be a ring of sunlight surrounding the dark silhouette of the Moon, resulting in an annular eclipse.  Annular eclipses are more frequent than total eclipses, because even at its average distance from the Earth, the Moon is not quite large enough to completely cover the Sun's disk.

Solar eclipses can only occur at the time of New Moon.  However at most New Moons, the Moon either passes above or below the Sun as seen from the Earth, hence the Moon's shadow misses the Earth.

A Lunar eclipse takes place when the Moon moves through the shadow of the Earth cast by the Sun.  Since the Moon shines by reflecting Sunlight, it is darkened during the eclipse.  The diameter of the Earth is nearly four times that of the Moon. The Earth's shadow is also larger than the Moon's and is large enough for the entire Moon to be immersed in it during a lunar eclipse.

When a Lunar eclipse occurs, it is visible from half of the Earth at any moment.  This contrasts with a Solar eclipse where the area from which a total or annular eclipse is visible is only of the order of a hundred kilometres across, although a partial eclipse will be visible over a considerably larger area, although still much less than half the Earth's surface.  As a result from any one place on the Earth's surface you will see many more Lunar eclipses than Solar eclipses.  Despite this, taking the Earth as a whole, there are more Solar eclipses than Lunar eclipses: in effect the Earth is a larger "target" for the Moon's shadow, than the Moon is for the Earth's shadow.

Lunar eclipses can only occur at the time of Full Moon. However, just as in the case of Solar Eclipses, at most Full Moons  the Moon passes either just above or below the Earth's shadow and so no eclipse occurs.

Next Total Solar Eclipse in Australia

Following the eclipse of 2002 observed from South Australia, the next total eclipse of the Sun visible from the continent will be on 2012 November 13. On that day a full total eclipse will be visible just after sunrise at Cairns.

To the west of Cairns, the Sun will rise already partly eclipsed. For observers in Arnhem land to the east of Darwin, across the Gulf of Carpentaria and the York Peninsula, the partly eclipsed sun will rise before totality. Further west still the Sun will also be partly eclipsed as it rises, but this will be after maximum eclipse.

Only near Cairns will the Sun rise just before the start of the eclipse so that all phases of the eclipse can be seen.

After Cairns the path of totality crosses the Tasman sea to the south of New Caledonia, just north of Norfolk Island, where the Sun will be 98% eclipsed at maximum. The path continues in a south-easterly direction passing between New Zealand and the Kermadecs, with North Cape New Zealand and the Kermadecs seeing a 91% eclipsed Sun.

The path continues in an easterly direction across the south Pacific but in no place will it again touch land. Cairns will be the only place on land to see full totality. Here the partial phase will start with the Sun about 1° up. Totality will last a second or two more than two minutes with the Sun at 13°.

The following year, 2013 May 10, a near sunrise annular eclipse will cross the York Peninsula a little to the north of Cairns.

Next Total Solar Eclipse in New Zealand.

New Zealanders will have to wait until 22 July 2028 for a total eclipse of the Sun.  Even then it will be very low in the sky.  The path of totality passes over the South Indian Ocean, but this time enters north Australia and passes diagonally across the continent with Sydney on the centre line.  Having crossed the Tasman Sea, the path crosses the Southern part of the South Island with Dunedin in the centre of the path.  The eclipse will occur there at about 4.15 pm and the Sun will only be about 8.5° above the horizon.

There are two further total eclipses for New Zealand within a decade, in 2037 and 2038. In addition annular eclipses cross the country in 2035, 2042 and 2045. Solar eclipses in New Zealand to 2050.

Conditions necessary for an Eclipse to Occur

The conditions for an eclipse to occur are similar to those for transits of planets across the Sun. That is at the time of New Moon (for Solar eclipses) or Full Moon (for Lunar eclipses)  the Moon must be close to one of the points (nodes) at which it crosses the ecliptic.  Unlike the ascending nodes of the planets which change in position only very slowly, the positions of the Moon's nodes effectively revolve round the Earth relative to the Sun with a period of just over 18 years.  As a result there is no fixed time of the year at which eclipses occur.

Since the nodes are revolving in a retrograde direction (opposite to the direction of rotation of the Earth about the Sun) the "eclipse year" is shorter than a calendar year at about 346.6 days.  Thus eclipses tend to occur a little earlier each successive year.  Because they must occur at New or Full Moon this is usually about 10 days earlier rather than 19.

Recurrence of Eclipses.  After 18 years (more accurately 6585 and a third days, i.e. 18 years and 11 and a third days) the Lunar nodes return to the same place and so similar eclipses occur.  This period of time is known as the Saros.  Because of the third of a day involved the positions of the eclipses on the Earth's surface move round by about 120° of longitude.  Thus similar eclipses recur at a similar position on the Earth after 3 Saros.  Even then there are changes to the latitude.