With winter (in the Northern Hemisphere) approaching, I thought it was about time I gave a summary of which planets are visible over the next few months. The longer nights, enabling easier viewing of the night-time sky, is one of the few pluses about this time of year as far as I am concerned. So, which planets are visible this winter (2015/16)?
The times I will give for various planets rising or setting are for Cardiff, where I live. So, if you are living elsewhere the times will almost certainly be different. Obviously, if you are living in the southern hemisphere you are about to move into summer not winter. But, although times may vary depending on your location; whether a planet is visible or not, and whether it is visible in the evening after sunset or in the morning before sunrise will not be different.
Of the 5 naked-eye planets, all but Saturn are visible this winter. Here is more detail about each.
Mercury is currently in Sagittarius, rising before the Sun and thus setting after the Sun. So, it is currently an evening object. It reaches maximum elongation on the 29 December when it will be to the East of the Sun, and on this day it will set in Cardiff at 17:43. The Sun sets on this day at 16:11 in Cardiff, giving some 1.5 hours after sunset to see Mercury. Although these setting times will vary depending on your location, what will not vary is the time between sunset and Mercury setting, which will be about 1.5 hours no matter where you live.
1.5 hours between sunset and Mercury setting it very good. Mercury is rarely this far from the Sun; so for those of you who have never seen Mercury, this month of December provides a very good chance. Find a view to the western horizon which is uninterrupted and away from city lights, and use the chart below to find Mercury. It will be reasonably bright, at a magnitude of -0.5.
Mercury will reach inferior conjunction on 14 January, whereupon it will reappear as a morning object later in January and February.
Venus is currently in Libra. It is a morning object, very bright before sunrise. At a magnitude of -4.1 you cannot fail to see it. It will reach maximum western elongation on 12 January. You can see it in the diagram below of the sky before sunrise, which also shows where to find Mars and Jupiter. Venus will be visible as a morning object throughout this winter and into the spring.
Mars is currently in Virgo. It is rising at the end of December just after 2am, so is a morning object. In fact, it can be seen in the morning sky along with Venus and Jupiter throughout much of the winter, as the diagram below shows. At the end of December it has a magnitude of +1.3, fainter than nearby Spica, which is at +1.05. Mars will reach opposition on 22 May, by which time it will have brightened to -2.1, so some 23 times brighter; making April, May and June by far the best time to see this planet.
Jupiter is in Leo, and is also currently a morning object. At the end of December it rises just before 11pm. It will be at opposition in early March (8 March), and so in late winter and spring it will be an evening object, but for most of this winter it is better seen in the morning before sunrise.
I like it when one can see Jupiter and Venus at the same time, as it allows one to see how much brighter Venus is than Jupiter. Normally Jupiter is the brightest point-like object in the sky, but when Venus is visible it outshines Jupiter by a factor of 6 or so.
Saturn is currently in Ophiuchus, and this winter is not the time to see Saturn. It will reach opposition in early June (3 June), so spring and summer are the best times to see Saturn in 2016 and over the next few years. It will not become a winter object again for another 14 years or so.