By Duncan Steel 23/05/2021


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Lunar eclipse visible from NZ on Wednesday

There will be a total lunar eclipse to be seen from throughout Aotearoa New Zealand on the evening of Wednesday May 26th, easily visible to all just as long as clouds do not intervene.

An eclipse of the Moon occurs when it passes close to opposite to the Sun in the sky, and so enters Earth’s shadow. While total solar eclipses – when the Moon comes between Earth and Sun – are rare events for any particular location on our planet, a total lunar eclipse can be witnessed from any point situated on the half of the globe in night-time.

For this Wednesday’s event, one might say that NZ is ideally located (see the map at the head of this post), with the eclipse commencing around 9pm, and totality lasting from 11:11 until 11:26, while the Moon is high in the sky. As I wrote above, so long as the clouds stay away, anyone should be able to watch the progress of the eclipse. No special equipment is needed, although if you have a telescope or a pair of binoculars it is interesting to witness the edge of the terrestrial shadow creeping across the lunar surface.

The vital times (all NZ Standard Time, NZST) are as follows:

Penumbral eclipse (partial shadow of Earth) begins at 8:47pm.

Umbral eclipse (shadow of the edge of the Earth) starts to cross the Moon at 9:45pm.

Totality (Moon completely within Earth’s shadow) begins at 11:11pm.

Totality ends at 11:26pm (i.e. totality lasts 15 minutes).

Umbral eclipse (shadow of edge of Earth leaves Moon) ends at 52 minutes past midnight (i.e. early Thursday morning).

Penumbral eclipse ends 1:49am (early Thursday morning).

A set of diagrams showing all sorts of information about this lunar eclipse is available here. The NASA website on eclipses (maintained by the wonderful Fred Espenak) has a set of information about other forthcoming lunar eclipses here.

A few words of explanation on the above. The penumbra defines an early (and late) phase of the eclipse, during which just part of the Sun is being obscured by the Earth, in terms of sunlight getting to the Moon. Therefore the Moon appears a bit darker, and progressively so as the penumbral phase advances. The umbra defines a complete shadow of the Earth: that is, all of the Sun is being covered by Earth, from the perspective of someone on the Moon. What happens is that one sees a ‘bite out’ of the lunar disk (the partial phase), getting progressively larger as the umbral stage goes on, until all of the Moon is within Earth’s shadow: that is the total phase.

If one thinks about the monthly lunar cycle (which actually lasts between 29.2 and 29.8 days, though the average is 29.53059 days: the synodic month), the Moon is generally brightest at full moon, which is when it is closest to opposite the Sun in the sky (astronomers would say it is 180 degrees away in ecliptic longitude) – except when a lunar eclipse occurs. Just before the penumbral eclipse commences the Moon is the brightest it can be, but then it gets darker because Earth is stopping some sunlight from reaching it, and so there is a lesser intensity to be reflected back to us. During totality, the Moon is the darkest it ever gets (if we ignore when it disappears from sight when close to the Sun in the sky, half a lunar month away).

During a total lunar eclipse the Moon does not go black, however. Rather it goes a dark brown, and some would say it is the colour of blood. The reason for this is that some sunlight leaks through our atmosphere, at Earth’s limbs (i.e. the circular edge of Earth as seen from the Moon). More of that sunlight leaking through is at the red end of the spectrum, for the same reason as the sky is blue: shorter wavelengths are more-efficiently scattered by atmospheric molecules, and therefore more light at the red end reaches the Moon.

 

Lunar eclipses in history

Different things interest different people. The thing that interests me most about eclipses is the fact that they occur in specific cycles, which enabled ancient astronomers to deduce the length of the year to phenomenal precision using multi-century records of lunar eclipses in particular. Also I find it fascinating how eclipses have affected history. That is, they have been turning points, prior knowledge (or seizing upon an eclipse as being a propitious sign) being used by some kings and generals to achieve advantages over superstitious foes.

Examples of this include the Second Battle of Syracuse in 414 BCE (which was pivotal in the eventual downfall of the Athenians and their overthrow by the Spartans), and the Battle of Gaugamela in 331 BCE (when Alexander the Great finally defeated King Darius III, thus conquering the Persian Empire).

Two decades ago I wrote a book about eclipses and in particular their historical significance, published in multiple editions.

 

 

Closer to the present day, in 1504 Christopher Columbus used astronomical tables showing forthcoming eclipses when he and his men were stuck on the north coast of Jamaica, trying to repair their worm-eaten vessels. The indigents had been feeding the Europeans for some months, and had enough of this imposition. The tale goes that Columbus invited the local leaders aboard, and told them that the Christian god was unhappy with their refusing to help further, and to show that god’s anger he would darken the Moon and turn it the colour of blood. The eclipse duly occurred as Columbus knew it would (in fact he used its time of occurrence to deduce the longitude of Jamaica), and the relatively-ignorant locals were terrorized. Columbus made a show of beseeching the god to make the Moon whole and bright again, the needed food was forthcoming, and eventually he and his men made it safely back across the Atlantic.

Because we are able to calculate the times of eclipses with considerable accuracy, they are also used to identify the dates of specific events in history. A prime example is the crucifixion of Jesus Christ. Comparison of the biblical accounts with darkness on that day (apparently due to a sandstorm) and the Moon being the colour of blood as it rose in the evening has led to the suggestion that the day was Friday April 3rd in AD/CE 33, though this is subject to much scholarly debate, as the hackneyed phrase goes. There would indeed have been a partial lunar eclipse in progress at moonrise for Jerusalem on that date.

Christopher Columbus cons the natives in Jamaica (drawn centuries later, and with some considerable artistic licence).

 

Rare solar eclipses, more-frequent lunar eclipses

I wrote earlier that total solar eclipses are rare events: if you remained at any particular geographical location, you could expect to experience a total solar eclipse about once every 500 years. A total solar eclipse paints a track across Earth’s surface that is typically 100-150 km wide, and if you are north or south of that track you experience only a partial eclipse. That is why some enthusiasts fly around the world in order to chase such events.

In 2002 I flew back from the UK to South Australia we see the total solar eclipse from near Roxby Downs in the Outback. In 2012 I drove from Canberra to Cairns to witness such an event from the Atherton Tablelands as the Sun rose above the Pacific.

Total lunar eclipses may be seen more often, though, because if you are anywhere in the hemisphere facing the Moon then you should be able to witness it. Even if there are a few clouds around, a lunar eclipse goes on for some time (totality in solar eclipses is rather briefer) and so you should be able to see what is going on.

Typically a total lunar eclipse may be seen from any particular location once every few years, but NZ is fortunate in this regard over the next decade. Another total lunar eclipse will be visible from here on November 8th next year; on March 14th in 2025; on March 3rd in 2026; and on the last day of 2028. There are several other partial lunar eclipses due too (i.e. when the Moon is not totally enveloped in Earth’s shadow), in particular on November 19th this year, when the Moon will come within a tiny sliver of being entirely immersed in our shadow.

 

Further resources

There are many astronomical websites that tell you more about lunar eclipses, and this one (Wednesday May 26th) in particular. A nice NASA webpage that has videos of what to expect is here.