Alan Clitherow is the Planetary Section Director of the Society for Popular Astronomy and an active member of DAS, and writes regularly on planetary observations. This article is reproduced from the Society For Popular Astronomy and with the kind permission of Mr Clitherow.
At the start of April Mercury is between the Earth and the Sun and thereafter emerges into the morning sky to give a very good apparition for the southern hemisphere. Greatest morning (western) elongation happens on the 29th of April with Mercury leading the Sun by 27 degrees. Unfortunately for northern UK latitudes the planet barely reaches 5 degrees of elevation by sunrise on that date; once found Mercury can be followed into full daylight as long as every precaution is taken to keep the Sun well away from telescope or finder fields of view. Look for it rising due East from 0415 UT a few days either side of the 29th of April. In May Mercury slides ever closer to the Sun, going on to solar conjunction in early June but southern observers should be able to follow it for at least the early part of May
Venus is an excellent evening object for both April and May and favours observers in the northern hemisphere. On April Fool’s Day look for it at sunset, 20 degrees up and due West; night-on-night it will then appear slightly higher at the same moment. By May 1st it will be 23 degrees up at sunset for mid UK latitudes and by months end some 25 degrees up. In the period it grows slightly in apparent size from 10 to 13 arc-seconds and its illuminated phase falls from 95% to 80% with its brightness a dazzling -3.9 magnitude, brightening further to -4.0 by the end of May.
Mars is a low altitude morning target for UK observers in this period and keeps very close company with Saturn for the first week of April. On the 2nd and 3rd of the month they are less than 1.5 degrees apart with Mars sitting just below Saturn, low in the south-east before sunrise. Once found the pair can be followed into daylight but will have only some 14 to 16 degrees of elevation as they transit due South. Because of the prism-like effects of our atmosphere when viewing such low targets fairly narrow band-pass coloured filters may help with the view; red or yellow for first choice. An Atmospheric Dispersion Corrector (ADC) is, however, a better choice for detailed observation under these circumstances.
As the period progresses Mars rises a little later than Saturn and follows it along a similar path however the elevation of either planet at transit remains low from the UK so there is little to be gained by waiting for later in the apparition. Mars grows from 8.5 to 15 arcseconds in apparent size in the period so good detail should be seen in steady seeing conditions.
Jupiter is a little better placed in the period and comes to opposition at midnight, Universal Time, on the 9th of May. Early in the period look for Jupiter rising in the south-east around 2225 UT and transiting at 0250 UT. By opposition night Jupiter is rising in evening twilight and can be followed all night; by the 23rd of May Jupiter transits at 2300 UT. At no time during this two month period will Jupiter transit at higher than 23 degrees of elevation for mid UK latitudes so, once again, an ADC will help greatly with detailed viewing and imaging. Its apparent size will be above 40 arcseconds throughout so a wealth of detail will be seen on good nights.
As mentioned Saturn keeps close company with, and then leads Mars across the sky as the period progresses. Once you have watched Jupiter transit due-south, transfer your attention to the south-east to follow Saturn as it rises. Of interest, on the 17th of April the ringed planet is at aphelion, its farthest point from the Sun in its 29.5 year orbit. Its ring system is tilted towards us by more than 25 degrees in the period and will appear almost as wide across as Jupiter, at just below 40 arcseconds in apparent size. If you have the opportunity this is a view not to be missed.
Uranus will be in conjunction, behind the Sun from our point of view, on the 18th of April and so is effectively unobservable throughout this period. Neptune was in conjunction in March so is emerging into the morning sky but even at the end of May will be a very difficult low altitude target before sunrise in the south-east. Both planets will be much better placed later in the year.
Society for Popular Astronomy
April & May 2018