The top sights to observe or image this month

DON’T MISS: Total eclipse of the Moon

BEST TIME TO SEE: The morning of 16 May, from 02:32 BST (01:32 UT) until moonset
RECOMMENDED EQUIPMENT: Naked eye

A total eclipse of the Moon occurs on 16 May. The Moon’s 5˚ orbital tilt normally has it passing above or below Earth’s shadow in space. It’s only when there’s a straight-line alignment between the Sun, Earth and Moon that the Moon crosses into Earth’s shadow. When this happens, the observers on Earth’s hemisphere facing the Moon witness a lunar eclipse. This is what will happen on the morning of 16 May, when we in the UK will be in the right hemisphere for the first half of the eclipse.

At the Moon’s distance, Earth’s shadow has two parts: a weak outer penumbra and a dark umbra. The penumbral shadow is the region where, if you were on the Moon looking back towards the Sun, you would see it partially eclipsed by Earth. At the outer edge of this shadow the partial solar eclipse would be tiny, increasing in magnitude the deeper into the penumbra you go. The umbral shadow sits at the centre of the penumbra and is the part of Earth’s shadow where all of the Sun’s light would be blocked from view. The darkness of the penumbra increases towards the umbra.

Earth casts an interesting shadow because it has an atmosphere. Light skimming the edge of our planet is refracted to partly infill the umbra. As our atmosphere scatters blue light, light that has passed through it tends towards the redder end of the spectrum, giving the umbra a red or orange colour.

Things kick off with the penumbral eclipse at 02:32 BST (01:32 UT). This is a weak part of any lunar eclipse and difficult to detect. As the Moon moves deeper into the penumbra the shadow’s depth increases, and you’re likely to see this as a dark shading near the western limb. It becomes darker until the Moon encounters the umbral shadow at 03:27 BST (02:27 UT). The umbral shadow is distinct and darker than the penumbra. The ensuing partial phase of the eclipse grows in magnitude until, at 04:29 BST (03:29 UT), the Moon is engulfed in dark shadow: this is the start of totality. 

All the while, dawn will be progressing to create an interesting conundrum. Whereas a bright non-eclipsed Moon is easy to see against bright sky, an eclipsed Moon against such a sky may seem to vanish from view, depending on the eclipse’s darkness. From the UK, the Moon sets just after the point of greatest eclipse, timed for 05:11 BST (04:11 UT).

May conjunctions

BEST TIME TO SEE: Mornings of 1–5 May, 18 May and 25–31 May
RECOMMENDED EQUIPMENT: Binoculars

There are several impressive conjunctions occurring this month. On 1 May, the bright planets Venus and Jupiter appear close in the morning sky. The position for this conjunction isn’t ideal as the pair remain low as dawn breaks. However, with a clear eastern horizon it should be possible to see them both. Their visibility is assisted by their brightness, Jupiter at mag. –2.0 and Venus at mag. –4.0. At 05:00 BST (04:00 UT) they appear 22 arcminutes apart, less than the apparent diameter of the full Moon. On subsequent mornings, both planets will appear to separate, with Jupiter appearing west of Venus. By 5 May, their apparent separation will have increased to 4˚, which is the equivalent of eight apparent diameters of the full Moon.

All the while, Mars will be heading in from the west. Before it meets up with Jupiter later in the month, Mars passes half a degree to the south of mag. +7.9 Neptune on the morning of 18 May. This meeting takes place under twilight conditions and in honesty, with low altitude it’s unlikely to be that easy to see. Perhaps this is an interesting exercise for a camera to try and record.

Having brightened to mag. +0.7, Mars meets mag. –2.1 Jupiter on the mornings of 25–31 May. Again, the dawn twilight will be brightening as the pair gain altitude, but their relative brightness should allow both planets to be seen fairly easily with the naked eye. In order to spot them, find a location with a flat, clear eastern horizon. They rise around 03:00 BST (02:00 UT). On the morning of 25 May, a 25%-lit waning crescent Moon sits to the south of Mars, adding an extra element to this show.

Lunar occultation of Porrima

BEST TIME TO SEE: 13 May from 01:30 BST (00:30 UT)
RECOMMENDED EQUIPMENT: Binoculars

On the morning of 13 May, the 87%-lit waxing gibbous Moon will occult the binary star Porrima (Gamma (γ) Virginis). The event occurs at 01:50 BST (00:50 UT) as seen from the centre of the UK, Porrima disappearing behind the Moon’s dark limb. This time may vary by up to a few minutes either side depending on your actual location. The best strategy is to observe from 01:30 BST (00:30 UT), first locating the Moon and then the star. This will ensure you don’t miss anything.

The Moon’s orbital motion keeps it moving east against the background stars. Eventually, it will have moved sufficiently far to allow Porrima to re-emerge from behind the Moon’s bright edge. This will occur at 02:42 BST (01:42 UT) as seen from the centre of the UK, and again, it’s wise to start observing 10-15 minutes earlier than this time to make sure you don’t miss it.

Porrima is a binary star with an orbital period of 169 years. Back in the early 2000s the apparent separation of the two components was so small that it was virtually impossible to split them. Now that situation has improved somewhat and the two similarly bright stars (mag. +3.6 and mag. +3.7), appear separated by a little over 4 arcseconds.