Shropshire Sky At Night – June

‘Summer’ officially starts following the solstice at 10.13am BST on June 21.

Noctilucent clouds, a spectacular phenomenon of the summer twilight sky.
Noctilucent clouds, a spectacular phenomenon of the summer twilight sky.

Our Earth rotates on its axis once each day, while at the same time orbits the Sun over the course of a year.

However, the axis of rotation of the Earth is not lined up with the axis of motion around the Sun. Instead, it is tilted at 23 degrees. This tilt means that during one half of the year the north side of the Earth is tilted slightly towards the Sun and the south is tilted away.

At the exact moment that the Sun is directly over the Earth’s equator as we start to tilt away from the Sun we experience the summer solstice.

Daylight, light pollution and clouds are the nemesis of every visual astronomer. However the twilight of a summer night sky can be adorned by a magnificent atmospheric phenomenon that is well worth seeking out. For about four weeks either side of the longest day, water vapour from the polar regions is carried to great heights by a surge of cold air.

Temperatures at this height (mesosphere) can drop to around minus 120C, producing ice clouds. Thirty minutes, or more, after sunset these noctilucent clouds show themselves as billowing neon blue ripples like sand left on a beach after an ebbing tide.

Noctilucent clouds, a spectacular phenomenon of the summer twilight sky.

On the 14th we are treated to a ‘super full moon’. A supermoon occurs when the full moon coincides with the moon's closest approach to Earth in its orbit. This is because the moon's orbit around the Earth is not a perfect circle. It has an average distance of 238,000 miles (382,900 km) from Earth, but its apogee and perigee, the closest and farthest approaches from Earth, change every lunar month.

Supermoons make the moon appear a little brighter and closer than normal, although the difference is hard to spot with the naked eye.  However given clear skies we should be treated to a spectacular sight after 10pm.

Planet hunters will have a hard time again this month with Mercury and Venus hard to spot even in the morning sky. Mars and Jupiter provide compensation with fine views in the pre-dawn sky, with Saturn the most captivating planet in the solar system being the first to rise before midnight.

Ursa Major (the Great Bear) is a worldwide favourite constellation, with its seven brightest stars referred to as The Plough, or in our modern times The Saucepan. It is usually the first star pattern that most people get to know.

Unlike most constellations the majority of stars in The Plough lie at the same distance from Earth and were born at the same time. Looking closely at the star at the middle of the saucepan’s handle will reveal that it is a double star. Mizar and its fainter companion Alcor (horse and rider) are one of the few binary stars that you can split with the naked eye, and are historically a test of eyesight – so good luck.

Steve Szwajkun FRAS, Shropshire Astronomical Society

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