At that precise moment, the sun reaches the southernmost point in its annual journey around the sky. This makes December 22 the longest day of the year. Here in Dunedin, we will get to enjoy 16 hours and 45 minutes of luxuriating in our notoriously warm sunshine.
With the longest day comes the shortest night. Despite its brevity, there’s a lot to see in the sky after sunset on Thursday. In particular, keen-eyed local stargazers with an unobstructed southwestern horizon can try to spot Venus and Mercury after sunset. What makes viewing the innermost planets quite an interesting thing to do now is that Venus is actually much further away from Earth than Mercury. This week the distance to Venus is more than 244 million km, while Mercury is "only" 150 million km away. Recent calculations by a team of experts have shown that while Venus can approach Earth much closer than Mercury, on average, throughout the millennia, Mercury is closer to Earth than Venus!
Viewing the two inner planets will not be easy. You will almost certainly need a pair of binoculars. Both planets are in the constellation of Sagittarius. Venus sets just an hour after the sun, while Mercury disappears below the horizon 20 minutes after its sibling. To find the planetary pairing, you need to be at your chosen location as the sun sets. Venus is so bright that it should be relatively easy to find, even in strong twilight. The second planet from the sun is 9 deg above the horizon at sunset. By 10pm when the sky should be decently dark, Venus will be 4 deg and, if conditions are good, just visible to the naked eye. Through binoculars or a telescope, you should be able to see the planet’s tiny 97% illuminated disk.
Once it is dark enough to see Venus, Mercury should also be visible. Look for a pinkish-coloured "star" some 5 deg above and to the left of Venus. Through a telescope, Mercury’s 67% disk will resemble that of the moon when at gibbous phase.
