Sky Matters - Looking ahead to the summer solstice

THE longest day of the year occurs on June 21 — the summer solstice.

Since the winter solstice on December 21, the sun’s maximum height has been rising each day until the summer solstice, after which it starts to reach less high until the winter solstice — when the sequence starts again.

The solstice is usually on June 21, but it can occur between the 20th and 22nd. For those of you who wish to watch sunrise on June 21, be aware that it’s at 5.02am and liable to be chilly if it’s clear.

At this time of year the colours of the landscape generally appear more vibrant than in winter. While this is due to a number of factors, one that is sometimes overlooked is the role played by our atmosphere. In the winter months the Earth’s atmosphere scatters the sun’s blue and yellow light away from us and so colours are more reddish, more muted. In the summer, the higher sun is less affected by the atmosphere and the blues and yellows rejoin the reds to restore a more complete palette.

The nights are at their shortest in the next month, but there is still plenty to see. Jupiter remains one of the brightest objects in the sky, visible in the west after sunset. Mars can be found almost due south around 11pm and burns bright red. Saturn is a short distance away to the west, about half as bright and yellow-ish in colour. If you have trouble finding Saturn, look for it on the night of June 18-19 when it is just below the full moon, although the moon may wash it out a little.

A few of the summer’s bright stars now start to make their appearance. In particular, the star vega can be seen high overhead around 11pm (and is visible throughout the night). vega is a young star, about a tenth the age of our sun, shining with an intense blueish-white colour. It has a disk of dust and gas around it that may well be forming planets right now. In a little over 11,500 years vega will take over from our current, rather non-descript pole star, having previously held that status in about 12,000 BC. If our ancestors then were navigating by the stars, they would have been fortunate to have vega as a bright pole star (see below). This changing of the pole star happens because the Earth’s axis wobbles rather like a spinning top (which is essentially what the Earth, or any planet, is).

The sky never gets fully dark in June and in many ways this is a good month to take a closer look at our nearest and brightest neighbour, the moon. It is widely believed that the moon resulted from a catastrophic collision between the Earth and a Mars-sized planet (Theia) about 4.5bn years ago. The small size of the moon means it has cooled to a stage where it has been mostly inert for billions of years, with not enough gravity to hold onto any atmosphere it may once have had. Many of the craters we see today were formed in the earliest, violent days of the solar system and have remained largely unchanged. The best time to observe the Moon in June is around the 12th (first quarter) or 27th (last quarter) — look along the line between the bright and dark halves for the finest views of craters and mountains.

DID YOU KNOW?

The Kepler Spacecraft, which is designed for planet hunting, recently discovered nine planets of similar size to the Earth that are just the right distance from their parent sun to allow water to exist as a liquid. Prime targets for life?

Dr Niall Smith is director of CIT Blackrock Castle Observatory in Cork. An astrophysicist, he is also head of research at CIT. If you have any astronomy questions for Niall, email [e-mail]