Mercury is at superior conjunction on the 3rd on the far side of the Sun and then emerges into the evening sky. It should be visible by mid-month at -1.1 magnitude low in the northwest in twilight. By the 31st it will set 2h after the Sun but will have faded to +0.4 magnitude. The very thin waxing crescent Moon will be only 26 hours old when it sets close to Mercury about 21h on the 17th. Given a very clear sky they should be visible to the naked eye, but binoculars may be needed to locate them. Venus is a brilliant -4.2 magnitude passing from Taurus into Gemini, setting in the northwest about 23h 30m throughout May. Moon close by on the 19th. Mars is 0.9 magnitude passing from Aquarius into Pisces during May though still in morning twilight. Moon near by on the 13th.

Jupiter is -2.6 magnitude and in Ophiuchus rising about sunset by the 31st. Moon near by on the 4th-5th. Saturn is 0.5 magnitude and in Leo, setting about 0h by the 31st. Moon close by on the 22nd. Uranus is in Aquarius and rises about 01h by the 31st. Moon near by on the 12th.

Neptune is in Capricornus rising about 0h by the 31st. Moon near by on the 10th.

The Moon: full 2d 10h, last quarter 10d 04h, new 16d 19h, first quarter 23d 21h.

The Moon is near Antares on the 4th, Regulus on the 23rd and near Spica on the 27th.

London area, sunset: 1 May 19h 25m, 31 May 20h 10m, sunrise: 04h 30m, 03h 50m. Nautical twilight ends: 1 May 20h 50m, 31 May 22h 00m, begins: 03h 05m, 01h 55m. Edinburgh area, sunset: 1 May 19h 50m, 31 May 20h 43m, sunrise: 04h 25m, 03h 34m. Nautical twilight ends: 1 May 21h 35m, 31 May 23h 53m, begins: 02h 45m, 00h 35m.

Mercury is the planet closest to the Sun and of the five naked eye planets the one that we see least often. Its mean or average distance from the Sun is 58 million km, with Venus at 109 and the Earth at 150 million km. It takes 88 days to orbit the Sun (Venus 225 days and Earth 365 days).

Like the other inner planet Venus, Mercury always appears in the general direction of the Sun and can never be high in the south late at night. The apparent distance of the planet from the Sun is called its elongation, east in the evening and west in the morning. For Venus the greatest elongation is 47? and as the orbit is almost circular this varies very little. Mercury has the most eccentric orbit of the planets and elongations vary between 18? and 28?. Its orbit is inclined about 6? to the Earth’s orbit so it can appear well above or below the ecliptic, the plane of the Earth’s orbit. These factors when combined with our view from the moving Earth make every apparition of Mercury different. Mercury is more easily seen from the tropics and southern hemisphere.

The period between superior conjunctions, which covers an evening apparition and a morning apparition, is called the synodic period and for Mercury averages 116 days (Venus 584 days). Near superior conjunction Mercury shows a full phase which decreases to a half or first-quarter phase near greatest eastern elongation after which it becomes a narrowing crescent towards inferior conjunction, then passing through the last quarter phase to full again. Mercury appears about 2.4 times larger at inferior conjunction than at superior conjunction, but as Mercury is rough and airless like the Moon, it reflects much less light towards us near the crescent phase.

The distances of Mercury from the Sun and Earth and the phase all affect the apparent brightness or magnitude and how easily it is seen also depends very much on its elongation from the Sun which determines how dark the sky is and of course we need a transparent sky to see it well. Near superior conjunction Mercury can reach -2.4 magnitude, an average brightness for Jupiter, but by greatest elongation this falls to around +0.5 mag and near inferior conjunction to a faint +5 mag though it is seldom observable then.

The current apparition begins with Mercury at superior conjunction on the May 3, at greatest eastern elongation (23.4?) in the evening sky on June 2 and at inferior conjunction on the June 28. Passing into the morning sky it reaches greatest western elongation (20.30?) on the July 20 with the next superior conjunction on August 15.

This current cycle between superior conjunctions lasts only 104 days with the previous four lasting 112, 106, 128 and 114 days.