Mercury to transit face of Sun on 9th
May is seldom an outstanding month for astronomy for observers at Scotland’s latitudes. The Sun’s northwards progress, welcome as it is, leads to later and briefer nights and, as the month ends, twilight begins to persist throughout the night even over the south of the country.
Two astronomical events occur this May, though, that should arouse our interest. The first is the transit by Mercury across the Sun’s face on the 9th which, if the weather holds, should be our best opportunity until 2049 to view its inky silhouette against the Sun. The month also sees Mars approach closer to us and appear brighter than at any time since 2005.
I must repeat the usual serious warning about the dangers of observing the Sun. To prevent permanent damage to your eyes, never look directly at the Sun through a telescope or binoculars, or even stare at it with the unaided eyes. We may project the solar image onto a shaded white card using a pinhole, binoculars or a small telescope, but note that using a large telescope for this may damage its eyepiece.
Sadly, Mercury’s outline will be too small to view by pinhole-projection, and nor will we see it using so-called eclipse glasses. In my opinion, though, it is best to equip your telescope with a certified solar filter to cover the objective (“big”) end of your instrument and block all the harmful radiation.
The precise times we experience the transit can vary by a couple of minutes across the Earth. For Scotland, Mercury begins to encroach on the eastern (left) edge of the solar disk at 12:12 BST and it takes a little more than three minutes before its outline is complete against the Sun. After crawling across the southern half of the Sun it finally leaves at the south-western (lower-right) edge at 19:41.
At a mere 12 arcseconds in diameter, though, it appears only 1/150th as wide as the Sun and a fifth as wide as did Venus during its transit in 2012. Whereas there are more than a dozen transits of Mercury each century, the next one by Venus is not until 2117 and we must be patient until 2125 for the next to be observable from Scotland.
The Sun climbs 7° northwards during May as sunrise/sunset times for Edinburgh change from 05:28/20:53 BST on the 1st to 04:36/21:46 on the 31st. The Moon is new on the 6th, at first quarter on the 13th, full on the 21st and at last quarter on the 29th.
Other than during its transit, Mercury is not visible for us at all this month, and neither is Venus which is lost in the glare on the Sun’s far side.
Jupiter, though, remains prominent in the heart of our southern sky at nightfall where it is slow moving in southern Leo and reaches a stationary point on the 10th before edging eastwards again. May has it dimming slightly from magnitude -2.3 to -2.0 as it recedes from 723 million to 791 million km and its disk shrinks from 41 to 37 arcseconds in diameter. Catch it near the Moon on the 14th and 15th.
By our star chart times, Leo and Jupiter are sinking into the west and our southern sky is dominated by the bright star Arcturus, the equally bright planet Saturn and, most conspicuous of all, the Red Planet, Mars. The Plough is tumbling westwards from the zenith as the Summer Triangle formed by the bright stars Vega in Lyra, Deneb in Cygnus and Altair in Aquila, occupies our lower eastern sky.
Mars rises in the south-east at 23:14 BST on the 1st and at sunset on the 22nd, the day it stands directly opposite the Sun in the sky at opposition. Our chart plots it low down in the south-south-east about one hour before it reaches its highest point, albeit less than 13° above Edinburgh’s southern horizon.
It lies 5° above the red supergiant star Antares in Scorpius at present but is retrograding, or moving westwards, against the stars and enters Libra towards the end of the period as it brightens from magnitude -1.5 to rival Jupiter at magnitude -2.0 at opposition. Because it is on the inwards leg of its somewhat eccentric orbit, it is actually closest to us at 75,280,00 km on the 30th, almost a million km closer than on the day of opposition.
Mars’ low altitude means that views of its reddish disk, only 18 arcseconds wide at opposition, may be less sharp (under “poorer seeing”) than if it stood high in the sky. Even so, telescopes should show the small northern polar cap, tipped 10° towards us, and other surface features as they drift slowly to the right across the disk. Those features return to almost the same position from one night to the next since Mars’ day is 40 minutes longer than that of the Earth. Mars comes 17 million km closer during its next opposition in 2018, but will be 4° lower still in our sky so we should make the most of any chance to view it this time around.
Shining to the east (left) of Mars is Saturn which this month brightens from magnitude 0.2 to 0.0 as it creeps westwards in southern Ophiuchus. It always rewards us with stunning telescopic views, its disk being (like Mars) 18 arcseconds wide but set within a glorious ring system that spans 41 arcseconds and has its north face inclined towards us at 26°. Look for the Moon above Mars late on the 21st and closer still to Saturn on the next night.