SkyMatters: When does summer really start?

IT has always been something of a mystery to me as to when “summer” starts, but the most commonly accepted definition seems to be on 1st June, in which case summer covers the months of June-July-August. 

This is slightly different to the astronomical definition of summer, which starts on 21st June on the Summer Solstice. 

This year the moment of solstice occurs at 4:54pm when the Sun will appear to be directly over the Tropic of Cancer. 

On this day the Sun is the farthest north that it will ever appear in the sky, reaching about 60 degrees above the horizon. The Solstice is also the longest day of the year, lasting a whopping 16 hours, 42 minutes and 46 seconds. 

But don’t despair that the days immediately either side are depressingly short. The length of the day on 20th June is a mere four seconds shorter than on the Solstice and half that on 22nd June. No need to plan for an early bar-b-q.

With the long days, and correspondingly short nights, the opportunities for getting spectacular views of the night sky are limited at this time of the year. 

At the latitude of Ireland the Sun does drop below the horizon, but not very far, and light reflected from the upper atmosphere ensures a glow in the sky throughout the night. This makes observing an even greater challenge than usual.

The reflected light lowers the contrast with which we see faint objects and washes out some that would otherwise be visible. 

Even in the darkest of our dark sky reserves in Kerry and Mayo, almost completely devoid of light pollution from cities and towns, our sky will not fully darken during June and July. 

By contrast, if we lived on the Moon, which has no atmosphere, once the Sun set the skies would appear perfectly black except for the stars and planets and glorious Milky Way stretching from horizon to horizon. 

Without an atmosphere there would also be no twinkling, but because each night on the Moon lasts 14 Earth days, we would also be able to observe the cosmos in its rawest, uncontaminated form without the need to rush.

 And yet without an atmosphere the temperature would plummet to -220 celsius at night or rise to +120 celsius during the lunar day. On balance, I reckon we can live with the observing restrictions imposed by our temperature-moderating atmosphere.

Curiously, the brighter skies make it easier to pick out certain groupings of stars, or asterisms, because we don’t get confused by the myriad of fainter ones. 

The best known asterism is probably the Summer Triangle, composed of stars from three different constellations. The stars, Vega, Deneb and Altair can be seen towards the east after dusk, gaining height as the night wears on.

They will be with us throughout the Summer and are a reminder for me of my boyhood days spent outside in my parents back garden peering skywards, just because.

On June 12 the largest of the planets in our solar system, Jupiter, will be closer to the Earth than at any other time in 2019. 

At a distance of 641 million km it will be almost seven times more distant than our Sun. Look to the south after twilight and it will appear as the brightest object in the sky. It will also appear to twinkle less than the stars nearby. 

This is because Jupiter appears as a tiny disc rather than a point of light and the Earth’s atmosphere has less of an effect on extended objects such as discs compared to points. Indeed once you get to know the difference between planets and stars your eyes pick up this difference quite readily and you can often tell a planet just from its lack of twinkle. 

Sometimes less is more.

Further information on the night sky is available on the Blackrock Castle Observatory website at www.bco.ie/sky-matters/

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