Look low to the south-southeast at 11:35pm BST tonight and, if clear, you will see tonight’s rising full Moon at the instant the 2016 summer solstice occurs in the Northern Hemisphere. The last time that this happened on the same UK day was 22 June 1967. For scale, this view is 60 degrees wide, or three spans of an outstretched hand at arm’s length. Note that Saturn and Mars are 18⅔ degrees apart this night. The full Moon has been enlarged for clarity. AN illustration by Ade Ashford.As any self-respecting druid or Sun-worshipper will know, 11:35pm BST on Monday, 20 June marks the Northern Hemisphere’s summer solstice of 2016, the instant that the Earth’s north pole is tilted at its greatest angle toward the Sun. It heralds the arrival of astronomical summer and the longest northern days, since the Sun is as high as it can get in the sky at local noon — and, let us not forget, the frustratingly short nights for keen observers or the 24-hour daylight for those north of the Arctic Circle.The obliquity of the ecliptic, ε, is the angle that the plane of the Earth’s equator (red line) makes to the plane of its orbit (dotted black line passing through the Sun). This is currently about 23° 26′. At the Northern Hemisphere summer solstice in June, the Earth’s axis (dotted and dashed black line) is tilted toward the Sun by angle ε such that the Sun is overhead at the Tropic of Cancer and doesn’t set north of the Arctic Circle. Six months later, the Earth’s axis is tilted away from the Sun by angle ε and it is the Northern (Southern) Hemisphere winter (summer) solstice, the Sun is overhead at the Tropic of Capricorn and doesn’t set south of the Antarctic Circle. Illustration credit: cmglee, NASA, Wikimedia.In the Southern Hemisphere, where the Earth’s south pole is of course tipped away from the Sun, today marks the winter solstice (though when it occurs is actually on 21 June, Australasian date). Here the days are shortest as the Sun appears lowest in the northern sky at local noon while nights are long to enjoy the sight of the Milky Way passing overhead — at least one could were it not for the full Moon in the sky!
Yes, today is not only the summer solstice for Northern Hemisphere observers, but the Moon is also full — the first time since 22 June 1967 that this has happened on the same day for observers in the British time zone. (Technically, the Moon reaches full phase at 12:02pm BST today.) So if the UK’s current monsoon can break long enough, look low to the south-southeast horizon at 11:35pm and toast both the start of astronomical summer and the full Moon.
Inside the magazine
Find out all you need to know about what is happening in the night sky of the coming month and how to observe it in the July 2016 edition of Astronomy Now.
A pockmarked, icy landscape looms beneath NASA’s Cassini spacecraft in new images of Saturn’s moon Dione taken during the mission’s last close approach to the small, frozen world. Two of the new images show the surface of Dione at the best resolution ever. Cassini passed 295 miles (474 kilometres) above Dione’s surface at 7:33pm BST on 17 August 2015.
Some of the final results sent back by ESA’s Venus Express before it plummeted down through the planet’s atmosphere have revealed it to be rippling with atmospheric waves and, at an average temperature of -157 °C, colder than anywhere on Earth. The aerobraking experiment has improved our knowledge of our planetary neighbour and holds great promise for ESA’s forthcoming ExoMars mission.
Observers in the British Isles looking due south close to 6pm GMT on Friday, 7 December will find magnitude +0.1 planet Mars about 30 degress, or a span and a half of an outstretched hand at arm’s length, above the horizon. What you won’t see unless you have binoculars or a small telescope is that magnitude +7.9 outermost planet Neptune lies just one-tenth of a degree from the Red Planet.
