Night Sky in May

The first half of May is the best time  this year to see Mercury. It is the closest planet to the sun, with a maximum angular separation of 28 degrees. This means it can never be seen in a dark sky, but when well placed east of the sun can be seen for an  hour or so after sunset. The combination of Mercury’s orbit and earth’s means that it has 3 periods of appearing each side of the sun per year, but the time of year makes a big difference. May for evening apparitions is good for 2 reasons. Mercury has an eccentric orbit, and May is one of the times when it is furthest from the sun, so the angular separation is near its maximum. Secondly, the angle that the ecliptic (the path through the sky apparently taken by the sun, moon, and planets) takes at the western horizon in early May is still quite steep, and so a relatively high component of mercury’s angular separation of the sun is upwards from our perspective, so making it easier to see.

The upshot is that if you look at the brightest part of the horizon half an hour or so after sunset during the first half of May, you should be able to pick up Mercury easily enough. It appears star like, but is brighter than almost all stars, so if you see a bright object in the right part of the sky, that will be Mercury.You will need a good western horizon, and binoculars will help. I usually find it easiest to spot it with binoculars first, then as the sky darkness it becomes easier to see with the naked eye, but not for long as it is also moving towards the horizon. (The Pleiades star cluster will also be close by at the start of the month) If you have a telescope, you will be able to see its phase like a tiny moon close to first quarter, but that close to the horizon you won’t see any detail.

Mercury is not the only planet on view. Venus shines brightly in the West (it is the brightest object in the sky after the Sun and Moon) after sunset, and again a telescope will show a phase like the moon. The disc will be much bigger than Mercury’s, and much higher in the sky. The uniform cloud cover offers little of interest for observing. I find the most interesting thing about Venus is trying to spot it as early as possible in the evening. Jupiter (the next brightest object in the sky), high in the south at sunset, is still well placed for observing. Finally, Saturn comes to opposition at the end of the month. It can be seen very low in the southern sky from around 11 pm. Saturn is also brighter than most stars in the sky, and shines with a steady yellow light. The star chart below shows where to find it then just above the sting tail of Scorpio, with the bright red giant star Antares.

Saturn at Opposition May 2015