If the sun was to disappear we wouldn’t know about it until eight minutes later

DURING March the bright stars of winter give way to the inexorable lengthening of the day as the sun rises higher in the sky. On March 20, the day and night are of equal length, the so-called spring equinox (‘equal night’).

If the sun was to disappear we wouldn’t know about it until eight minutes later

For March, the planet Jupiter is an easy target for the naked eye observer, rising around 7pm almost due east.

It is the brightest object in the night sky, apart from the moon.

Through a pair of binoculars (or telescope) you can see that Jupiter is flanked by between one and four points of light that move over a period of a night or so — these are the four Galilean moons discovered by the famous astronomer Galileo Galilei in 1610 soon after he turned the newly invented telescope to view the skies.

Follow a line to the right of Jupiter and you will come to the brightest star in the sky, Sirius, almost exactly due south at around 8pm. Sirius is 70 degrees away from Jupiter.

A handy trick to use to measure degrees is to note that the outstretched palm of the hand is roughly 10 degrees across for all of us! The light from Sirius has taken 8.6 years to reach Earth.

Compare this to the time taken for the light from the sun to reach us — eight minutes. Bizarrely, that means we are seeing the Sun as it was eight minutes ago and Sirius as it was 8.6 years ago, or, looking at it another way, if the Sun was to disappear we wouldn’t know until eight minutes later.

If you look closely at Sirius you will see it twinkle, whereas Jupiter generally does not. Twinkling is caused by turbulence in the Earth’s atmosphere and has nothing to do with the stars or planets themselves. Twinkling is a bit of a nuisance to astronomers and one of the main reasons why observatories are built high up on mountains, above as much of the atmosphere as possible.

Having found Sirius, now look another 20 degrees to the right and upwards by 20 degrees and you can’t miss the three stars in the belt of the magnificent constellation of Orion, the Hunter. This is one of the night sky’s truly spectacular sights and it’s with us for another couple of months before it can no longer be seen because of the lengthening day.

The right-most star in Orion’s belt is called Mintaka and it is truly huge, about 24 times more massive than our sun and 190,000 times brighter. It only looks as faint as it does because of its distance of 1200 light years.

The surface of Mintaka is so hot that if we replaced our sun with Mintaka all life on Earth would be eradicated with UV radiation. One might imagine that such a large star (and it’s by no means the largest) would last a very long time, but in fact big stars burn brightly but have short lives. Mintaka’s lifetime is a mere 17m years.

Compare that to our sun, which has a lifetime of 10,000m years. Put another way, when the dinosaurs roamed the earth some 65m years ago, Mintaka wouldn’t have yet been born.


The star on the upper left of Orion is called Betelgeuse. It has a distinct red colour. Betelgeuse is a massive star, about eight times larger than the sun, which is near the end of its life. It has expanded to an incredible size of 821m km and is thought to be a prime target to explode as a supernova within the next million years or so, lighting up the sky brighter than the full moon for a period of weeks.

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 skymatters@examiner.ie

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