Dundee Astro - Planetary Notes

Venus continues its magnificent evening apparition with its greatest eastern elongation from the Sun, some 45 degrees, on the 4th of June. Around this date look for it first appearing around 25 degrees high in the west shortly after sunset and follow it until around local midnight as it sinks to the north-west. At magnitude -4.4 it will be very bright, massively outshining the nearby stars Castor and Pollux. Overnight on the third into the fourth of June the visible phase of Venus should be exactly 50% illuminated (dichotomy), however it is well known the apparent moment of 50% illumination is usually observed some days earlier for evening apparitions, that is, when the visible phase steadily falls (wanes) through the apparition; the opposite is true of morning apparitions. While very variable, and personal to individual observers, it is usually agreed that visual dichotomy will be about 4 days early; around the first of June. This is an example of the well known Schröter effect.

As the period progresses the angle the ecliptic makes with the horizon falls with each sunset, dragging Venus ever lower in the sky even as Venus itself sinks slowly back towards the Sun. It will still be observable but with increasing difficulty as we move into July and will be lost before the end of the month unless sought-out in daylight.

Mars is a faint and distant early evening object in this period, losing altitude in the west with each new sunset. Throughout June, Venus and Mars move steadily closer with Mars slightly higher and to the left (south) of Venus. Early-on the gap is around 10 degrees but this falls to less than 5 degrees by the start of July; thus Venus can be used to find the fainter planet for much of the period. The gap between them starts to open slowly after the first week of July. If you want to see the Red Planet it is best to do so early in the period when it shines at magnitude +1.6 and its 93% illuminated disc will span just 4.7 arcseconds (4.7”). By late July Mars is effectively unobservable.

Jupiter is a poorly placed morning object early in this period but improves steadily. At the start of June it rises a little north of east in morning twilight and reaches only 10degrees of elevation in the east by sunrise. By early July Jupiter rises around 00.45UT for mid-UK latitudes and will still be obvious more than 20 degrees high in the east some two-and-a-half hours later. At magnitude -2.2 and 36.5” in apparent size it should show considerable cloud detail and can easily be followed into daylight for observation at higher altitudes. Late in the month Jupiter rises around 23.00UT and will be visible nearly 45 degrees up in the south-east by morning twilight. It will have grown to nearly 40” in size across its equator.

Saturn rises ahead of Jupiter throughout this period so should be better placed in morning twilight, but this is slightly cancelled by its elevation which is going to be lower when compared with its neighbour. In early June Saturn rises a little south of due-east around 01.00UT and is visible some 12 degrees up in the south-east by the time Jupiter rises, but it is soon after lost as the sky brightens. By early July Saturn rises at 23.00UT and can be followed to, perhaps, 25 degrees of elevation in the south-south-east. Late in the month Saturn rises in late evening twilight and will transit, due-south a little after 02.00UT, around 27 degrees up for mid-UK latitudes. It brightens slowly from +0.9 to +0.6 in the period and ends it with the ring system spanning over 43” in apparent size. The rings themselves appear quite ‘closed’ to us with just 7.3 degrees of tilt our way in mid-June, then opening slightly for the rest of the period.

Uranus is lost to the dawn for UK observers in early June, sitting a little above Mercury. It does improve into July, rising around 01.00UT at the start of that month, some 12 degrees left (north) of Jupiter. At magnitude +5.8 it will be hard to find in the pre-dawn skies but will be easier by the end of the month, rising around 23.15UT and a couple of degrees closer to Jupiter. It will then reach some 35 degrees of elevation a little south of due-east by twilight, roughly midway between Jupiter and the Pleiades star cluster.

Neptune rises in Pisces around 01.30UT in early June for UK observers, but at Magnitude +7.8 will be very hard to find in the brightening skies. Look for it from early July when it rises, due-east, from 23.30UT, 20 degrees left (north) of Saturn and then follows Saturn as it rises towards the south. By late July Neptune can be found around 30 degrees high in the south-east at 01.30UT, lagging Saturn by 21 degrees.