Dundee Astro - Planetary Notes

Mercury starts April as a very low morning object, rising directly below Venus and a little behind Saturn, but will be hard to see in the brightening sky. It improves only slowly and by the end of the second week may be caught some 7 degrees below and a little north (left) of Venus, almost due-east, before sunrise. It is also very close to Neptune on 16 and 17 with Neptune just 2/3rds of a degree above Mercury so, in theory, 4 major planets will sit in the field of view of modest binoculars in this period very shortly before sunrise. In practice Neptune will be be lost due to the bright sky and both Mercury and Saturn will be hard to see from the UK.

Mercury stretches to Greatest Western Elongation (GWE) on 21 April then slides steadily back towards the Sun. In early May, the dawn rise of the ecliptic helps keep Mercury visible above the horizon at sunrise but only in brightening twilight and it slips into solar conjunction at the very end of the month.

Venus rises around 04.30UT in early April and reaches some 10 degrees of elevation in the east by sunrise. At magnitude -4.2 it will be very bright but at just 4% illuminated it will show the thinnest of crescents. As the period progresses Venus stretches slowly further from the Sun and rises around 03.15UT mid-period and at 02.15UT late in May. It slips below the ecliptic on 08 May; this somewhat counters the steady rise of the ecliptic through the period as night turns to day. Therefore it never reaches great elevation, being perhaps 12 degrees high at sunrise for mid-UK latitudes by the end of the period. It will, however, have grown to 48% illuminated phase as it approaches its own GWE on 01 June.

Mars is a reasonable evening object in this period. Early in April it will be visible some 60 degrees high in the south as twilight falls and can be followed into the early hours, being above 30 degrees of elevation in the west a little before midnight UT. By mid period it will first appear a little under 50 degrees high in the south west and will sink to 30 degrees high in the west by 22.30UT. In late May, Mars will first appear 30 degrees high, a little south of due-west shortly after sunset. In the period its disk shrinks from 8.2 arcseconds to 6 arcseconds (8.2” to 6”) in apparent size and its brightness from +0.45 to +1.22 so it is best observed as early as possible in the period to see the most detail.

Jupiter should be observed as soon as evening twilight falls in early April. It sits well ahead of Mars along the ecliptic and first appears perhaps 45 degrees high in the west-south-west during the first week of the month. It wont then set until midnight UT but its period at a useful elevation for observation is becoming limited. Mid period it will first be visible due-west but below 30 degrees high, a little after sunset, so detailed observation will be becoming difficult. Late in May it will be too close to the Sun for safe observation. When first seen in twilight it will be very obvious, at magnitude -2.1, with a disk some 36” in apparent size across its equator. This will fall by early May but will still be be -1.9 and 34” respectively, so much may still be observed until then.

As mentioned, Saturn is an early morning object, barely visible in April ‘though close to Venus, Mercury and Neptune. In very early May it sits just below and right of Venus, which can help in locating it, and can be followed into daylight for viewing at higher altitude and in steadier air. The rings will be some 38” across, making it a large target, but very nearly edge on from our point of view. Interestingly Earth has moved from viewing the ‘top’ or northern part of the rings, to viewing just below them, while the Sun itself is still above the ring plane. This means we see the shadowed side of the rings and they are noticeably darker than one might expect. On 06 May the Sun also crosses below the ring plane so the rings, although still very narrow from our point of view, will considerably brighten; a rather rare event. By late May, Saturn rises in nautical twilight, around 02.00UT, and some 25 degrees ahead of Venus which rises 15 minutes later. Again, it should be followed into daylight for better views at higher altitudes.

Of the Ice-Giant planets Neptune is extremely hard to find in bright morning twilight. If you can find Mercury in mid-April you may, just, be able to see Neptune nearby. Uranus starts April in the evening sky but will also be hard to find. On 01 April it sits 6 degrees below the narrow crescent Moon, just after sunset and almost due-west, but will be very low by astronomical twilight. This situation worsens as Uranus moves towards solar conjunction in the last week of