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An astronomical Olympic challenge

As we enjoy the efforts of our athletes at the Rio Olympics, it seems appropriate that, this week, the sky gods are setting Otago stargazers an extraordinarily difficult celestial challenge to test our astronomical skills.

Tonight the moon is full at 9:27pm. A few minutes earlier at 9:24pm what might be the shortest, and most difficult to see, penumbral eclipse of the moon in recent history, will begin.

Readers are probably familiar with a total eclipse of the moon which occurs when Earth passes between the sun and the moon. This is an event that stargazers anticipate for months; the sight of the bright full moon turning blood red over a few hours as the moon crosses the darkest part of the Earth’s shadow is certainly one of the most awesome natural phenomena.

During a penumbral eclipse, the Earth blocks some of the sun's light from directly reaching the moon's surface, and as a result covers a small part of the moon with the outer part of its shadow, also known as the penumbra. For this week’s eclipse, if an astronaut were standing on the surface of the moon, she would see the edge of the sun barely touch the limb of the Earth.

The eclipse is predicted to begin at 9:24pm, when the Earth enters the penumbral shadow. Maximum eclipse will be at 9:44pm and the moon leaves the Earth’s shadow at 10:01pm, thus the eclipse will be over after just 37 minutes. Because the eclipse is so brief, and so little of the sun will be covered by the Earth, it’s very unlikely that any dip in light will be noticed by the human eye; that’s why if it’s clear, I hope to take photographs to see if the eclipse is visible at all. 

There’s actually intense debate in the astronomical community as to whether this is an eclipse or not. The eclipse is so marginal that the models used by some authorities are not predicting an eclipse at all; this is because the models all use different radii for the Earth and moon.