It’s the beautiful clear morning of 6 June 2012 here in Armagh and the Sun is rising into a cerulean blue sky.But look closer, and you can see a tiny circular black speck on the face of the Sun.This is a transit of Venus, one of the rarest spectacles in astronomy when our closest planetary neighbour crawls slowly across the Sun’s disc.Miss it if you dare as there won’t be another until 2117!Here are the basics of this rare planetary alignment.

image of 2004 Venus-Transit

The 2004 transit was captured from the unique perspective of NASA's Sun-observing TRACE spacecraft in ultraviolet light.(Image credit:NASA/LMSAL)


1.Why is this transit happening?When the planet Venus passes directly between the Sun and Earth, becoming visible against the Sun’s face, the planet is said to be transiting the Sun.During the transit, Venus is seen from Earth as a small disc of darkness moving across the blazing disc of the Sun.Venus is just a little smaller than the Earth but is far away, so it appears to travel slowly over the face of the Sun.Transits of Venus are events which last for hours (the last transit back in 2004 lasted six hours)

2.When does a transit of Venus happen?Transits of Venus are among the rarest regular astronomical phenomena, occurring in a pair separated by eight years, then another pair of transits 121.5 years later and another pair 105.5 years after that, then the pattern repeats.  The last transit of Venus took place on 8 June 2004.  Before that, the previous pair of transits were in December 1874 and December 1882 (this transit was observed from Armagh by John Dreyer and inspired a Sousa march which was lost for a century but rediscovered in time for the 2004 transit).

image of 2004 venus path

The path of Venus across the Sun in 2004 as seen from Slovakia (image credit:VT-2004 Team, Slovak Union of Amateur Astronomers Observatory Rimavska Subota via ESO)


3.Since Venus orbits between Earth and the Sun, it ought to pass in front of us regularly, why don’t we get a transit of Venus every few years?That’s a good point but it’s not quite so simple.The orbit of Venus around the Sun is slightly inclined to that of the Earth.Every 1.6 years Venus does indeed come between the Sun and the Earth but from our perspective it is usually “above” or “below” the Sun at these times.Transits happen when the geometry is just right for Venus and the Sun to be perfectly lined up from our point of view.

4.When exactly is the next transit of Venus?The 2012 transit begins on 5 June at 22:10 UT (Universal time, 11.10pm BST) and ends on 6 June at 04:50 UT (5.50 am BST) .The total duration will be about 6 hours 40 minutes but the exact duration varies by ± 7 minutes depending upon your location on Earth.For observers in the UK, Her Majesty’s Nautical Almanac Office (what a splendidly archaic name!) has a useful page with a guide to the transit with timings for selected UK cities including Belfast. If you live elsewhere, the US Naval Observatory has a page which can calculate the timings for your geographical location.

image of 2012 transit visibilty

Geographic visibility of the June 2012 transit (image credit:Fred Espenak/ NASA GSFC)


5.Where can I see it?The best views will be from the Pacific Ocean. North Americans will be able to see the start of the transit, while the inhabitants of South Asia, the Middle East, and most of Europe will catch the end of it. The transit will not be visible in most of South America or western Africa. Alas the transit will not be visible in its entirety from the British Isles as they are rather too far west, making the time interval between sunrise and the end of the transit rather short.  Here, the Sun will only be rising through the latter stages of the transit.

6.How can I see it safely?Staring directly at the Sun with the unprotected eye will lead to serious and probably permanent eye damage.The safest way to observe the transit is to project the image of the Sun onto a screen through a telescope, binoculars or even just a pinhole in a card, but if you must look at it use filters specifically designed for solar viewing, such as an astronomical solar filter or eclipse viewing glasses.

Image of venus transit

The Venusian atmosphere is refracted into a ring of light in this image from 2004.(Image credit:Lorenzo Comolli via ESO)


7.What is the scientific significance of a transit of Venus?In the past accurately observing a transit was vital for establishing the scale of the Solar System.The only way to measure the distance from Earth to the Sun (the Astronomical Unit) was by making precise observations of the tiny difference in the start and end times of the transit from widely separated points from across the Earth’s surface.The distance between the observing locations on the Earth was then used as a triangulation baseline to calculate the distance to Venus and the Sun.In the 17th century European astronomers made some epic journeys to make these observations.

Nowadays transits of Venus are less fundamentally important but some astronomers still find them useful.This time astronomers are planning to use this rare event to test techniques for studying  the atmospheres of exoplanets (planets orbiting other stars .Astronomers can already observe exoplanets transiting their stars (in fact this how many of these distant worlds are discovered).The researchers are confident they could observe telltale signatures of atmospheres containing life-supporting gases such as oxygen and water vapour in these transiting exoplanets but to be sure the plan is to calibrate their methods as Venus transits the Sun (since we already know what is in the Venusian atmosphere).

8.If I miss it when do I get another chance?After 2012, the next transits of Venus will be in December 2117 and December 2125, so set your alarm clock for the big morning!


ajit kanshide · May 16, 2012 at 12:51

Excellent information. How to project using binocular and pin hole semonstration guideline if would have been there will be better. Thank you

– Ajit

June 2012: What's in the night sky? | Astronotes · October 15, 2013 at 03:01

[…] in astronomy! For those seeking a more detailed in depth look at the Transit, why not check out the ‘8 Facts You Need To Know About The Transit Of Venus’ on the Armagh Planetarium […]

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