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Eclipses

An eclipse is what happens when a celestial body either enters the shadow of another celestial body or is obscured from the viewer by another celestial body. The most notable eclipses involve the Sun, Earth and the Moon, and are of two types – solar eclipses and lunar eclipses – depending on whether the Moon or Earth (respectively) is casting the shadow.

Solar and lunar eclipses occur every year. There are between 2 and 7 eclipses each year. If there are only 2 eclipses there will be one of each type. In 1934 there were 5 solar and 2 lunar eclipses. In 1973 there were 4 solar and 3 lunar eclipses, which won't happen again until 2094.

In 2011 there were 4 partial solar eclipses and 2 total lunar eclipses.

In November 2012 there was a solar eclipse visible from New Zealand. More details below.

Measuring an eclipse

Not all eclipses are total eclipses, and in the case of solar eclipses any particular location may experience less than the maximum eclipse on offer. The maximum amount of an eclipse is measured in several ways.

From NASA's Glossary of Solar Eclipse Terms:

Eclipse magnitude is the fraction of the Sun’s diameter occulted by the Moon. It is strictly a ratio of diameters. By convention, its value is given at the instant of greatest eclipse.

Eclipse obscuration is the fraction of the Sun’s area occulted by the Moon.

Eclipse magnitude and obscuration may be expressed as either a percentage or a decimal fraction (e.g., 50% or 0.50).

The magnitude is a simple way of describing the amount of eclipse but it can give an innaccurate idea of the strength of the eclipse for partial eclipses. For example, in a solar eclipse when the Moon has moved half way across the Sun's diameter the magnitude is 0.5, but less than 40% of the Sun's disc has actually been covered.

  
Magnitude Obscuration %
1.00 100
0.95 93.6
0.90 87.3
0.85 81.0
0.80 74.7
0.75 68.5
0.70 62.4
0.65 56.4
0.60 50.5
0.55 44.7
0.50 39.1
0.45 33.7
0.40 28.5
0.35 23.5
0.30 18.8
0.25 14.4
0.20 10.4
0.15   6.8
0.10   3.7
0.05   1.3
0.00   0.0

Solar Eclipse

A solar eclipse occurs when the Moon passes between Earth and the Sun, thereby totally or partially obscuring the image of the Sun for a viewer on Earth. A total solar eclipse occurs when the Moon's apparent diameter is larger than the Sun, blocking all direct sunlight, turning day into darkness. Totality occurs in a narrow path across the surface of the Earth, while a partial solar eclipse will be visible over a region thousands of kilometers wide.

The Sun's distance from the Earth is about 400 times the Moon's distance, and the Sun's diameter is about 400 times the Moon's diameter. Because these ratios are approximately the same, the Sun and the Moon as seen from Earth appear to be approximately the same size: about 0.5 degree of arc in angular measure.

The Sun and the Moon both vary in their apparent size because the Moon has an elliptical orbit around Earth and Earth has an elliptical orbit around the Sun.

An annular eclipse occurs when the Moon is far enough from Earth on its elliptical orbit that it doesn't completely cover the Sun. There are more annular eclipses than total eclipses. Because the relative sizes of the Sun and Moon are continuously changing, it's possible for an annular eclipse to change to a total eclipse or vice versa during the eclipse, so people in different parts of the planet will see a different sort of eclipse. This is called a hybrid eclipse.

2011 features 4 partial solar eclipses, on 4 January, 1 June, 1 July, and 25 November. The partial eclipse on 25 November 2011 will be visible at sunset from some of New Zealand – the further south the better.

Dunedin will experience a total solar eclipse on 22 July 2028. The path of totality also crosses Sydney, Australia.

The total solar eclipse on the afternoon of 13 July 2037 will cross the North Island of New Zealand. Auckland will get to 98.5%. The path of totality will include Waitomo, Te Awamutu, Taumaranui and Taupo, and cross directly over Puketiti Station and Napier. Another total solar eclipse occurs the following year on 26 December 2038. The path of totality passes over Levin and Palmerston North.

Solar eclipse 14 November 2012

The total solar eclipse on the morning of 14 November 2012 (NZDT) will be visible from New Zealand, but the path of totality is to the north east of the country.

Maximum eclipse over New Zealand will occur around 10:30 AM NZDT (21:30 UTC). Auckland will get to around 85% obscuration (magnitude 0.87), Waipu to 87%, Kerikeri to 89%, and Hicks Bay on East Cape to 87.6%.

Note that dates and times in the animation on the left are Coordinated Universal Time (UTC). Add 13 hours for New Zealand Daylight Time (NZDT).

The eclipse was easy – and safe – to view by projecting an image of the Sun onto a white screen using a pair of binoculars. To make the image stand out better I clothes pegged curtains around the binocular lens. A few pinhole sized openings resulted in their own images – an effect that can also be seen under trees, where their leaves result in speckled light, each one a crescent shape.

These photos show the view in Auckland at 10:13am and 10:29am, which was at about peak eclipse. Lesson learned from the second photo – don't aim through a window at a steep angle. The glass produces too many distortions.

Lunar Eclipse

A lunar eclipse is what happens when the Moon enters Earth's shadow.

In more detail, a lunar eclipse occurs when the Moon passes behind Earth so that Earth blocks the Sun's rays from striking the Moon. This can occur only when the Sun, Earth, and the Moon are aligned very closely with Earth in the middle. Hence, a lunar eclipse can only occur during a full moon.

During a lunar eclipse the Moon looks red because of light refracted through Earth's atmosphere.

The total lunar eclipse that took place on 16 June 2011 (NZST) was unusual in that the central point of Earth's shadow crossed the Moon. (Normally the Moon passes through one side of Earth's shadow.) The next time this will occur will be in 2018 but that eclipse will not be visible from New Zealand.

There are different sorts of lunar eclipses, depending on how complete the eclipse is. Wikipedia: As viewed from Earth, the Earth’s shadow can be imagined as two concentric circles. As the diagram illustrates, the type of lunar eclipse is defined by the path taken by the Moon as it passes through Earth’s shadow. If the Moon passes through the outer circle but does not reach the inner circle, it is a penumbral eclipse; if only a portion of the Moon passes through the inner circle, it is a partial eclipse; and if entire Moon passes through the inner circle at some point, it is a total eclipse.

2011 features two lunar eclipses, on 16 June and 10 December 2011. From New Zealand most of the eclipse will be visible, with the end obscured by moonset. Western Australia is an ideal viewing location, with the eclipse occurring during the evening.

Lunar Eclipse 16 June 2011

The lunar eclipse that occurred this morning was the longest eclipse in a decade. Martin Bush, curator of Melbourne Planetarium said "it would be the longest one since [July] 2000 – which was one of the three longest since 1000 BC." The eclipse lasted 100 minutes (the eclipse in 2000 lasted 107 minutes) and the next such eclipse is forecast to take place only in 2141. Early risers in the far south of New Zealand had a reasonably good view, but unfortunately it wasn't very visible in Auckland because it started just before dawn.