Christmas Eve pre-dawn vista

Ian Griffin
Ian Griffin
At 5.19pm on Sunday, December 22, the sun reaches the southernmost point in its apparent annual journey around the sky.

Astronomers call this instant the summer solstice. In the southern hemisphere, this means we experience the longest day and, correspondingly, with merely 8hr 14min of darkness, the shortest night of the year.

With so little darkness to enjoy, stargazers must make the most of these short December nights to get their summer extra-terrestrial entertainment.

Luckily, in the run-up to Yuletide, there are some splendid celestial scenes to enjoy for early-rising Otago skywatchers. In particular, I would like to draw your attention to what I hope will be a lovely sight an hour before sunrise on Christmas Eve.

Now I know that people are busy at this time of year, and asking you to get up at 4.30am is not going to put me high on your Christmas-card list. However, when you learn that there is a chance to spot a beautiful crescent moon, I know you will be jumping out of bed pronto to revel in the sight.

On Christmas Eve morning, the moon is located in the constellation Libra, the scales. It is below and to the right of my two favourite stars, called Zubenelgenubi and Zubeneschamali. The moon rises just after 4am. Once it clears the horizon, its bright crescent, which will be 7% illuminated, should present a delightful sight low in the eastern sky.

As the moon gets higher in the sky, you will see the darker portion of the lunar disc illuminated by earthshine. This ghostly effect is caused by sunlight reflected off both Earth and the moon before reaching your eyes.

The moon forms the lower apex of a triangle which includes the planet Mars and Antares, the brightest star in the constellation Scorpius. Interestingly, Antares is a Greek word which means “rival of Mars,” and the relative closeness of the two objects this week does allow us the chance to make a comparison.

This is my final column for 2019. I wish you all a starry Christmas and clear skies for 2020!

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