A celestial hat-trick

A celestial hat-trick

Radmila Topalovic (Royal Observatory, Greenwich) looks forward to a rare treat – three phenomena all linked to our only natural satellite

Stages of the eclipse in Universal Time (to convert to BST add 1 hour). Graphic: Finbarr Sheehy

This month we will be treated to a total lunar eclipse, a Harvest Moon and a supermoon – a hat-trick of celestial phenomena all linked to our only natural satellite.

A lunar eclipse occurs when the Moon lies directly behind the Earth, with the Sun on the other side.

We experience at least two lunar eclipses a year, and they occur due to the 5 degree tilt of the lunar orbit relative to the flat orbital plane of the Earth around the Sun, called the ecliptic.

When the Moon is close to the intersection of its orbital plane and the ecliptic we see a lunar eclipse.

Christopher Columbus used astronomical tables to predict the lunar eclipse of 1 March 1504 and used this knowledge to persuade the natives of Jamaica to continue to provide him and his crew with food, telling them that the eclipse was a sign from God.

We can watch our very own lunar eclipse without fear of God’s wrath in the early hours of 28 September.

The show will start with a full moon rising in the east at around 18:30 on the 27th, eventually entering the darkest part of Earth’s shadow called the umbra at 02:07 BST.

Totality begins at 03:11 and finishes at 04:23, resulting in the finale, another partial eclipse which will end at 05:27. Throughout the show, the Moon will be hurtling around the back of the Earth at a speed of 2,300mph.

The full moon will still be visible after it has entered the shadow of the Earth. It will turn a majestic shade of coppery red due to refraction of sunlight through the Earth’s atmosphere on to the lunar surface.

December 2010 lunar eclipse
The December 2010 lunar eclipse. Photograph: John Vermette/Wikimedia Commons

The red colour is due to the filtering effects of the atmosphere – we see a blue sky because the atmosphere scatters blue light but allows red light through. This is the reason why every sunrise and sunset turns a fiery red colour.

At the autumnal equinox on the 23rd we will experience equal hours of day and night and we will also see the Moon around the same time in the evening for several days.

During this time it is a so-called Harvest Moon – farmers once used this phenomenon to harvest their crops after sunset. On average, the Moon rises in the east 50 minutes later each day, but during the Harvest Moon this difference reduces to 30 minutes. This is due to the narrow angle of the ecliptic relative to our horizon.

The eclipse will also be a supermoon. Due to the elliptical nature of the lunar orbit, the distance between the Earth and the Moon is not constant, and on the 28th the Moon will be at perigee – its closest point to the Earth in 2015.

At this time the Moon will be more than 30,000 miles closer than its furthest point (its apogee) and it will appear almost 14% bigger in the sky – perfect timing for the lunar eclipse.

This article titled “A celestial hat-trick” was written by Radmila Topalovic (Royal Observatory, Greenwich), for The Guardian on Sunday 13th September 2015 20.30 UTC