The top sights to observe or image this month
Don’t miss: Perigee full Moon and the ‘Moon illusion’
BEST TIME TO SEE: Evenings of 14–16 June
RECOMMENDED EQUIPMENT: Naked eye
The lunar orbit is elliptical and over time this means the distance between Earth and the Moon varies. As you would expect, the apparent diameter of the Moon varies over its orbit because of this distance change. Apogee describes when the Moon is furthest from Earth, perigee when nearest. When it’s closest, the Moon’s apparent size is around 34 arcminutes, while at apogee the Moon’s apparent size is around 30 arcminutes.
Apogee and perigee have no connection with lunar phase, the orbit cycle being out of sync with the phase cycle. A full Moon near perigee gains more attention than a regular full Moon because it appears larger and brighter than average, and has also become known by the popular term ‘supermoon’.
However, if you compare a perigee full Moon to the ones in previous and following months you probably wouldn’t notice any difference.
Last month, the Moon was full at 05:15 The Moon’s apparent diameter changes in its monthly cycle, as shown here in a comparison over 11 days from full Moon diameter of over 33’ 40” on that date. In June, full Moon occurs at 12:52 BST (11:52
UT) on 14 June, perigee at 00:23 BST (23:23 UT) on 15/14 June, the Moon’s apparent diameter reaching 33’ 56”, fractionally larger than May’s. July’s full Moon occurs on 13 July at 19:38 BST (18:38 UT), perigee on the 13th at 10:05
BST (09:05 UT), resulting in a maximum apparent diameter of 33’ 57”. June and July’s full Moons also offer the chance to see the ‘Moon illusion’. This optical effect occurs when fuller phases of the Moon appear close to the horizon, after moonrise or before moonset. This month’s full Moon sets around 04:10 BST (03:10 UT) on the 14th, as seen from the UK’s centre. It can be seen rising again at 22:30 BST (21:30 UT) on the same date.
Catch it right and the illusion is powerful: it’s hard to believe that the Moon you’re looking at isn’t larger than normal. In reality, the low Moon is a bit smaller than it appears when high up due to increased distance and atmospheric refraction.
If you see the full Moon illusion, take a photo using a 200mm or longer lens. When it has climbed higher, do the same thing. Using a layer-based editor, put both Moons side by side. This proves the ‘giant’ Moon is no larger than a regular one.
Noctilucent cloud season begins
BEST TIME TO SEE: 90–120 minutes after sunset or before sunrise
RECOMMENDED EQUIPMENT: Naked eye
The season of noctilucent clouds (NLCs) is upon us once again, providing astronomical entertainment despite a reduced period of darkness at this time of year. NLCs are high-altitude ice clouds which form in a narrow layer, 82km up in the mesosphere. If present as the Sun dips below the horizon, they are able to reflect sunlight at their extreme altitude despite it being dark for us on the ground. As a consequence, NLCs appear to glow against the darker, deep twilight background during the summer months.
Typically seen 90–120 minutes after sunset above the northwest horizon, or at a similar time before sunrise above the northeast horizon, it’s worth bearing in mind that NLCs don’t always play by the rules. Also bear in mind that NLCs, if present, are typically low and can be easily hidden behind houses, trees and hills. Although it’s generally the case that they appear low, they have been reported at high altitude too. As mentioned, they don’t always play by the rules!
Typically NLCs track from northwest to northeast, disappearing in the middle of the night. However, extensive displays may persist all night long, with the bright region starting in the northwest, tracking through north and ending in the northeast. Under unusual circumstances NLCs have been seen away from their expected horizons, appearing low above the north and northeast even at sunset.
Taken all together, it’s clear that observing NLCs is an important task. Documenting their appearance, location and apparent brightness is useful as long as you also record the date, time and location. NLCs are a photogenic phenomenon, but again it’s important to make sure all the useful observational details are also recorded when you take photos of them.
Morning Venus and the Moon
BEST TIME TO SEE: 26 June, from 50 minutes before sunrise
RECOMMENDED EQUIPMENT: Naked eye
Venus is a morning planet in June (see page 48 for more details) and it also has a beautiful encounter with a 7%-lit waning crescent Moon on the morning of the 26th. On this date the pair will appear separated by 2.2˚ in the dawn twilight. However, as the day progresses the distance continues to reduce, albeit only slightly, which gives an opportunity for spotting Venus in daylight. (For more information about this see this month’s ‘Sky Guide Challenge’.)
Venus is a superb planet at any time, but even more so when it can be seen against a dark sky. Unfortunately, this won’t be the case during June and the best that’s on offer is to spot it against the deep blue of a late June dawn twilight. The presence of the Moon will enhance the scene and will add its own beauty by displaying earthshine, the effect which makes the night side of the Moon weakly visible.
The closest approach between Venus and the crescent Moon occurs just before 07:00 BST (06:00 UT), when both objects will be 1.9˚ apart, as measured from the centre of the Moon’s disc. This occurs after the Sun has risen, providing a great opportunity to locate Venus during the day.