Looking Forward; April and May 2019.
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.
© Alan Clitherow. All rights reserved
Society for Popular Astronomy
Mercury starts this period visible in the pre-dawn sky and is well placed for southern observers, but from UK latitudes the planet rises only 30 minutes before the Sun and will be hard to spot in the growing glare. On the 2nd of April Mercury is some 10 degrees east of Venus which, itself, sits just above the narrow crescent of the rising Moon and this may help locate Mercury just pre-dawn. Mercury and Venus grow slowly closer until the 29th of April when some 6 degrees separate them, thus Venus may be used as a daylight marker to find Mercury; after this Mercury is effectively lost, moving into superior conjunction, behind the Sun, on the 21st of May.
Venus itself is still a bright early morning object but sits slightly below the ecliptic which has a shallow angle with the morning horizon at this time of year. As a result Venus stays quite low and is best followed into daylight (as long as suitable safety precautions are taken with regard to the Sun) which then allows Venus to be observed at somewhat higher elevations. As April progresses into May daylight observation is helped by the improving angle and subsequent rise of the ecliptic, morning on morning, but the separation of Venus from the Sun is itself reducing, increasing the risk of daylight observation. Most observers will cease observations in mid or late April.
Mars is in Taurus during April and sets around 2300 UT initially, then a little earlier each night as the month progresses. This means it can be followed from evening twilight into full darkness, first appearing a little south of due-west and above 30 degrees of elevation. At magnitude +1.6 and vividly red it should be easy to find but with an apparent size now below 5 arcseconds surface detail will be hard to make out. Having said that on a clear day the air can be very steady shortly after sunset so major albedo features should be identifiable and imagers may see surprising levels of detail. By early May Mars will have drifted into Gemini and will first appear a little north of due-west after sunset, but still more than 20 degrees above the horizon for UK observers. Although it will then appear ever lower in the west, Mars can easily be observed in twilight to the end of the month.
Jupiter is a morning object in this period, rising around 0100 UT for mid-UK latitudes in early April and transiting, due-south in morning twilight, around 0500 UT. As mentioned the ecliptic is very low at this time of year so the elevation of Jupiter at transit is only around 15 degrees above the horizon, improving noticeably with more southerly observing latitudes. At a brightness of magnitude -2.3 Jupiter will be very obvious and at 40 arcseconds in apparent size across the equator considerable detail may be on view in its turbulent atmosphere. An atmospheric Dispersion Corrector (ADC) will help noticeably with both visual and photographic observations.
By early May Jupiter is rising from around 2320 UT and transiting at 0305 UT, still in darkness however its elevation at transit will not have improved. It continues to brighten and grow in apparent size reaching magnitude -2.6 and 46 arcseconds across by the end of May. At that time it will be observable from 2130 UT until full dawn however its elevation at transit will still be frustratingly low for UK observers.
After watching Jupiter rise and then transit, switch to Saturn which follows Jupiter across the sky throughout this period. Saturn follows a near identical path to Jupiter, just a little less than two hours behind it as the pair crosses the lower part of the southern sky; this separation changes only slightly throughout April and May. The angle between Saturn, the Earth and the Sun forms a right-angle (90 degrees) on the 10th of April and with Saturn appearing to the west of the Sun, this is event is known as West Quadrature. Saturn brightens a little, from +0.5 to +0.3 in the period and also grows a little in apparent size. Its ring-structure is tilted some 24 degrees towards us giving an open view of the northern hemisphere of the planet and, given good seeing, an excellent view of the rings themselves. As with Jupiter an ADC is recommended to gain the sharpest views.
Uranus starts April in close conjunction with the Sun so is unobservable. It may be glimpsed late in May as it starts its new apparition as a morning object. On the 18th and 19th of May it sits little more than 1 degree north of Venus which may act as a marker to find it around that time.
Neptune is near by Venus in early April. In fact on the 10th of April it is very close by indeed, sitting only 13 minutes of arc north of Venus on that morning. At magnitude +8.0 it is very likely to be lost in the glare from Venus but planetary imagers equipped with larger chipped cameras, such as the ASI178 or its various equivalents, might like to consider it a challenge to capture them both together on that date!