The Night Sky in June
June and July are testing months for stargazers. The June solstice, which this year occurs at 16:54 on June 21, represents the exact moment when the apparent motion of the Sun against the background stars reaches its most northerly extreme. The consequence of this is that at night, the Sun never dips that far below the northern horizon. From the centre of the UK, the centre of the Sun only disappears to a depth of 13 degrees below the northernmost point on the horizon in the middle of the night.
Twilight is the term used to describe the sky when the Sun has either just set or is just about to rise. It literally means “second-” or “half-light” and refers to the effect of sunlight scattering in the atmosphere when the Sun is below the horizon. Technically there are three main types of twilight. Civil twilight occurs when the Sun is less
than six degrees below the horizon. A degree is more-or-less equal to the width of your little finger at arm’s length, equivalent to twice the apparent width of the Moon.
Next is nautical twilight, a darker period that occurs when the Sun is greater than six and less than 12 degrees below the horizon. Finally, astronomical twilight marks the transition between nautical twilight and true astronomical darkness, a point when the sky cannot get any darker. Astronomical twilight is the time when the Sun is between 12 and 18 degrees below the horizon. At the June solstice, from the centre of the UK, the Sun just scrapes the brighter boundary of astronomical twilight. Further north, the sky never really gets properly dark at all, remaining at nautical twilight during the darkest part of the night.
Despite the lack of true darkness, there are still astronomical treats to be had. The brighter stars and constellations are still recognisable along with the planets and Moon. Then there is the phenomenon of Noctilucent Clouds, or NLCS. These “night shining” clouds thrive in twilight conditions and are rather spectacular if present but they can be difficult to predict.
NLCS are ice clouds that form in a thin layer within the mesosphere at a height around 82km (51 miles). They reflect sunlight but are too delicate to be seen until the sky has reached the end of civil twilight. If present they shine with reflected sunlight against the darkened twilight sky and often take on a gorgeous electric blue colour with delicate netlike structures.
Look out for them an hour to an hour and a half after sunset low in the northwest, or a similar time before sunrise low above the northeast horizon.
The evening twilight has another object of interest this month, the planet Mercury. Despite being a main planet, Mercury hasn’t been seen by that many people because it never strays that far from the Sun. This month, it reaches greatest eastern elongation, the point when it appears furthest from the Sun in the evening sky, on June 23. Prior to this, on June 17 and over the course of the following evenings, Mars and Mercury will appear close together. On June 18, the apparent separation between both worlds will be just 14 arcminutes, or roughly half the apparent diameter of the Moon. Look for them low above a flat northwest horizon after sunset.
On the evening of June 16, the full Moon will appear very close to Jupiter. Both rise together, forming an impressive sight. As the Moon comes up above the horizon it may look huge due to an effect known as the Moon illusion. The effect can be very convincing but it is just an illusion.