Royal Astronomical Society of New Zealand Transits of Mercury and Venus

The two inner planets, Mercury and Venus, occasionally transit across the face of Sun and can be seen, with suitable equipment, as a black dot on the Sun.  Such transits are fairly rare : for Mercury a number occur each century, but for Venus they are less frequent, pairs of them occur separated by 8 years, but the pairs are themselves separated by either 105.5 years or 121.5 years. 

Table of Transits of Mercury and Venus in the 21st Century

Where two dates are given the first is the UT date, the second the date in New Zealand

Two transits each of Mercury and Venus occur in the first 12 years of the 21st century. The second transit of Mercury, occurred on 9 November 2006 and, unlike the May 2003 transit, was fully visible in New Zealand. The next transit of Mercury to be fully from New Zealand is in 2052, on November 9, starting about midday and ending late afternoon. This is better placed than the next, 2065 Nov 12 which starts just after sunrise. Both are also fully visible in Australia.

The second transit of Venus for the century occurs on 2012 June 6. It is fully visible from New Zealand, starting soon after 10 am and ending very close to sunset. Being a winter transit the Sun will be fairly low in the sky.

The following transit of Venus will be not be until 2117 December 11, both New Zealand and Australia will be well placed for the event.

Transits were important as the first accurate method of determining the distance to the Sun.  A transit will start and end at slightly different times when viewed from different places on the Earth.  By timing the events from various places on the Earth the "parallax" and hence the distance of the Sun can be determined.  More accurate methods are available now, but careful measurements in the 18th and 19th centuries gave distances to within 1% of that currently accepted.

For Australia and New Zealand the most famous example of this was in 1769 when Captain Cook in Endeavour was in charge of an expedition to observe the transit of Venus from Tahiti in June of that year.  Later in November the expedition observed a transit of Mercury from Mercury Bay in New Zealand during the exploration of the coast line of the country.

Viewing a Transit

Safe viewing methods must be used, either by projecting the image of the Sun onto a suitable screen, or by using a suitable, specially designed, Solar filter in front of the telescope.

Any attempt to view the Sun directly could result in instant blindness.  It is NOT safe to use a filter at the eyepiece as the focussed heat from the Sun could shatter it: sun-glasses are certainly NOT suitable.  If unsure consult your local astronomical society about safe methods of viewing.

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Transits of Mercury in the 21st century

Transits of Venus in the 21st century

The transit in 2004 occurred on June 8 and lasted for over 6 hours.  It was not be visible from New Zealand, apart from the very beginning at sunset which was visible only from the northern tip of Northland and the extreme southwest coast of Fiordland. For both of these localities the sun had just about have set before Venus has fully moved onto the solar disk.

Australia saw the start of the transit but the Sun had set before its end.  The best placed countries for viewing the entire transit was India and Pakistan, the countries of the "middle east", east Africa and most of Russia.

The 2012 transit occurs on June 6 and will be visible from New Zealand, starting in the morning and ending very close to sunset.  Eastern Australia will also see all parts of the transit.  Being mid winter for the Southern hemisphere the Sun will be low.  The best places for viewing this transit will be eastern parts of Asia.

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Transit of Mercury, 15/16 November 1999

The transit was unusual in that Mercury crossed the Sun very close to the Sun's limb.  For places south of a line running from the southern part of the North Island of New Zealand across the Tasman to Brisbane and then over Australia to the extreme north-east of Western Australia there was only be a partial or grazing transit..
 

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Conditions Needed for a Transit to Occur

The plane of the Earth's orbit round the Sun is known as the plane of the ecliptic.  Since we are on the Earth, the ecliptic is the apparent path followed by the Sun through the stars.  The orbits of other planets round the Sun are tilted at small angles to the ecliptic and hence planets will usually be either above (north) or below (south) the ecliptic.  Transits of the Sun will occur if the inferior conjunction occurs within a day or two of the date at which the planet crosses the ecliptic.

The position of a planet on the ecliptic round the Sun can be measured in terms of its longitude.  The SUN is defined to have a longitude of zero at the March equinox, that is when the Sun crosses the equator moving to the north, about March 21.  From the point of view of the Sun, the EARTH will have a longitude of 180° at this time.  The Earth has a longitude 0° at the September equinox, about September 23.

The orbit of Mercury crosses the ecliptic at a longitude (measured from the Sun) of just over 48° as the planet moves from south to north.  This point is known as the ascending node.  The descending node (north to south) is 180° from this, i.e. just over 228°.  The Earth is at these longitudes on its journey round the Sun about November 11 and May 9 respectively.  Hence transits of Mercury will only occur close to these dates.

The ascending node of Venus is at a longitude of nearly 77° and the descending node is at nearly 257°.  The Earth is at these longitudes about December 9 and June 8.  So transits of Venus must occur close to these two dates.

No transits can occur for the outer planets, however from their points of view transits of the Earth (and any other planet nearer the Sun than them) may occur.  Thus on 11 May 1984, the Earth was in transit of the Sun for observers on Mars.  The Earth took about 8 hours to go across the face of the Sun with the Moon about 6 hours behind, so that for a while both bodies would have been seen as black dots crossing the Sun - if you had been observing from Mars. Also in August each year from 2000 to 2006, the Earth has made a transit of the Sun as seen from Neptune.

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