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Sat, 25 Mar 2017 - Looking Forward; April and May 2017.

Start the month by catching Mercury after sunset on the first of April when the innermost planet will be just past its greatest eastern (evening) elongation from the Sun. Trailing the Sun by 19 degrees this is the best evening apparition of the year for UK observers and Mercury can be found by looking just slightly north of due-west, around 1900 UT, and scanning the sky with binoculars around 14 degrees above the horizon (from mid-UK latitudes). Shining at a magnitude of +0.05 on that date, Mercury will show a 40% illuminated phase around 7.7 arc-seconds in size from pole to pole. Using a telescope with an atmospheric dispersion corrector and a red filter may help to reveal visible albedo features and these may be caught photographically with infra-red pass filters and planetary cameras.

The planet can be followed at the same time each night until the 11th of April, after which it will be below 10 degrees of elevation shortly after sunset and won’t be visible to the naked eye until around 1930 UT, and then at just 5 degrees of elevation. Mercury passes into inferior conjunction between the earth and the Sun on the 20th of April and then becomes a morning object. It reaches its greatest western elongation, rising some 26 degrees ahead of the Sun on the 17th of May but this will be rather poorly placed for observation from the UK. On that date Mercury will rise at 0338 UT but at a very shallow angle to the horizon and will reach barely 5 degrees of elevation as the Sun breaks the horizon. Observers in the southern hemisphere will be much better placed to observe this western elongation.

Venus starts April just past inferior conjunction so is rising only barely ahead of the Sun in the morning sky. It will be bright, at magnitude -4.2, but very low when it becomes visible, however it is worth seeking out to catch its large and slender crescent, around 50 arc-seconds from pole to pole. Look for it from around 0500 UT at the start of the month, only slightly north of due-east, and follow it day-on-day as it rises higher before each sunrise. Venus will reach its brightest in the last week of April, when it will shine at a dazzling magnitude +4.6 and show around a 25% illuminated phase, but it will have fallen noticeably in size to around 40 arc-seconds.  By the end of May Venus may be best seen from the UK around 0330 UT, due east and some 10, degrees above the horizon. Like Mercury, Venus is best observed from the southern hemisphere in late springtime but if you are up early it is well worth seeking out this brilliant beacon in the morning sky.

Mars is still visible in the early evening sky at the start of April. After you have found Mercury, move some 12 degrees higher in the sky and 10 degrees south to find the little bright spark of the red planet. With an apparent visual size of only 4.2 arc-seconds and shining at magnitude +1.47 there will not be much detail on view but this is the season for Martian dust storms and well-equipped imagers may be able to tell if such storms have started or even if they have become planet-wide as occasionally occurs at this point in the Martian seasons.

The prograde motion of Mars, west to east against the background stars, means that Mars maintains its visibility for an extended period and can be followed from dusk into the mid evening on subsequent days without it losing a lot of altitude day-on-day. Look for it at around 2030 UT on the 21st of April when it will sit some 3.5 degrees south and east of the Pleiades star cluster. By the start of May, Mars is setting around 2200 UT, some 2 hours after the Sun but fading in brightness towards +1.7 magnitude and it will be below 4 arc-seconds in size; by the last week of the month the planet is lost to evening twilight.

Jupiter comes to opposition on the 7th of April so is the standout -object for observation in this period. On that date Jupiter will be due south at midnight UT and at around 32 degrees of elevation; it can be followed from rise to set for a period of nearly 10 hours. Around this time it will shine at magnitude -2.4 and show a disc some 44.3 arc-seconds across at the equator of the planet so plenty of detail should be on view. As the period moves on Jupiter is more readily accessible in the mid-evening, transiting due south at 2230 UT on the last day of April and at 2130 UT by mid-May. By the end of May Jupiter is well past due-south as true darkness falls but at 2200 UT on that date the planet is still around 30 degrees of elevation in the south-western sky and well worth observing. Its size will have fallen a little to 41.1 arc-seconds and its brightness to magnitude -1.9 but plenty of detail will be on view in its highly dynamic atmosphere.

On the 4th of April, from 2203 UT and until 0012 UT on the 5th, the shadow of Io leads the moon itself, both visible against the clouds of the North Equatorial Belt; with shadow and moon appearing to just touch as they transit. Contrast this with the 9th of April when, from 2350 UT to 0216 the following day, both Europa and its shadow transit across the cloud tops of the northern temperate region of Jupiter with Europa now leading its shadow by less than a diameter of the moon itself. These transit events clearly show the ‘before’ and ‘after’ of opposition, when the Earth sits exactly between the Sun and Jupiter.

Saturn is a lovely morning object, rising in full darkness a little after 0100 UT at the start of the period, and transiting due-south around 0520 UT, in morning twilight, having climbed to perhaps 16 degrees of elevation. By mid-April Saturn transits an hour earlier and by the start of May can be found due-south at 0110 UT, still at 16 degrees of elevation. This low elevation is disappointing but observations in steady air will still show spectacular detail in the ring system which will be tilted towards us by 26 degrees and will stretch to an apparent size of 40 arc-seconds. On the night of the 9th to the 10th of May Saturn sits just below the full Moon and accompanies it all night. By the end of the month Saturn transits at 2300 UT and will continue to be observable well into the next period

The outer ice-giant planets of Uranus and Neptune are not well placed in this period with Uranus in solar conjunction on the 14th of April and unobservable until well into May; even then it will not rise until a little after midnight ut and will reach barely 20 degrees of elevation before being swamped by morning twilight. Neptune is only slightly better placed and on the last day of May it rises around 2315 ut and reaches around 25 degrees of elevation in the south-east by twilight at 0240 UT. Both these planets will be significantly more observable as late spring moves into full summer.


Added by: Alan Clitherow