Conjunction compunction
A night under the stars with some planets and the moon
Tuesday 13th July was a blue sky day here in New Zealand’s deep south. More importantly (at least for this keen stargazer) the sky looked like it would remain clear after the sun went down, meaning conditions would be perfect to observe a close approach between the planets Mars and Venus. Astronomers call these celestial close approaches conjunctions, and they can be quite fun to observe. On Tuesday Venus and Mars angular separation was roughly thirty minutes of arc, which is the same size as the full moon appears on the sky.
Both planets would be visible low in the NW sky after sunset. Along with the crescent moon, the conjuncting planets were located in the constellation Leo.
Although Mars and Venus appear to be close to one another in the sky, this is just a line of sight effect. The planets are actually separated by more than one hundred and fifty-eight million kilometres, with both being on the “opposite side” of the sun to Earth as can be seen from the diagram below.
As the sun went down, I set up my cameras and a telescope on my deck and waited for the sky to get dark enough to start photographing the conjunction.
Initially the planets were not visible in the bright twilight. However, the waxing crescent moon certainly grabbed my attention in the North western sky.
In deepening twilight, the dim glow of earthshine could be seen between the twin horns of the lunar crescent. Earthshine is sunlight that has reflected off both Earth and the moon before hitting your eyes. It is sometimes called the Da Vinci glow since the mechanism by which the darker portion of the moon is illuminated was first described by Leonardo da Vinci in the codex Leicester in around 1510.
Of course because Leonardo made his moon observations in the Northern hemisphere, his crescent faces the opposite direction to the one we see here in New Zealand. On Tuesday evening, the moon was just 3.1 days past new and its waxing crescent was a little over 10% illuminated.
As the sky darkened, first Venus and then Mars became visible, below and to the left of the moon.
At magnitude -3.9 Venus was approximately 190 times brighter than Mars, whose magnitude was 1.8. Then colours of the planets were a nice contrast, Venus was essentially white, while Mars was distinctly orange.
I have to confess to being transfixed by the celestial scene as the two planets slowly sank towards the Northwestern horizon. The night was still, and despite the cold, I kept snapping away with my cameras, obtaining some remarkable shots as the planets descended.
My enthusiasm for observing on this fine evening did not disappear when Mars and Venus set. I pointed my telescope to try and get some pictures of earthshine, for which I have a real fascination.
This turned out to be a lucky decision. The sky was really transparent, and as the moon sank lower, I noticed that it would probably set quite close to the brow of one of the distant hills, making for some pretty photographs.
Luckily the angle of the hill meant that most of the bright lunar crescent disappeared before the part of the moon illuminated by Da Vinci’s glow.
A pretty night indeed! And as noted in today’s title, I have no compunction about encouraging you to observe the next conjunction….. :-)
Until next time…. mā te wā