Transits of Planets

Transits of Venus occur when the planet crosses the face of the Sun as seen from the Earth. With suitable eye protection it can then be seen as a black dot on the face of the Sun. It takes a few hours for Venus to move across the Sun. Such events are fairly rare, at present transits of Venus occur in pairs separated by 8 years, with pairs of transits themselves over 100 years apart.

Mercury is the only other planet that can transit the Sun as seen from the Earth.

In recent years the presence of planets orbiting other stars have been detected by the slight reduction of light from the star as the planet transits across it.

Conditions Needed for a Transit to Occur

Obviously when a planet transits across the face of the Sun as seen from the Earth, the planet must be between the Earth and Sun and the three bodies must be almost exactly in line.

This condition can be broken down into three parts:

1. The planet must be between the Earth and Sun: this means it is an inferior planet with an orbit closer to the Sun than the Earth's orbit. Only Venus and Mercury meet this condition, transits are not possible for outer planets.

2. The three bodies must be almost exactly in line:

2a. The transiting planet must be in the same direction as the Sun in an east to west sense. This occurs when the planet is at conjunction. When the planet is between the Earth and Sun it is at inferior conjunction. (A superior conjunction takes place when the planet is at the opposite side of the Sun compared to the Earth.). As most conjunctions the planet would appear to pass either above (north) or below (south) the Sun, so no transit occurs.

2b. So, at a transit the planet must also be at the same height as the Sun in the north to south sense. This occurs when the planet is close to a Node, which is the point where its orbit round the Sun crosses the ecliptic, the plane of the Earth's orbit round the Sun. When at a node the planet is level with the Sun, so is on the ecliptic. This occurs twice in each orbit of the planet round the Sun. Usually when the planet is at a node it is not in the same direction as the Sun.

The orbit of Venus is inclined at an angle of 3.4° to the ecliptic, that is to the Earth's orbital plane. Venus crosses the ecliptic, so is at a node, twice in each of its orbit. The ascending node is when it crosses the ecliptic from south to north, and the descending when it crosses from north to south.



The diagram shows the orbit of Venus relative to the Earth's orbit round the Sun. The inclination of Venus's orbit to the ecliptic, 3.4°, has been exaggerated for clarity. The angles round the Earth's orbit shows longitude measured round the Sun from the first point of Aries. The two nodal points of Venus's orbit are marked with the dates the Earth is at the same longitude each year.

The Transit Window of Opportunity

Transits can only take place when both the Earth and Venus are close to one of the nodes in December or June. For the June node, a transit must occur within about 44½ hours of the time at which Venus is at the node, a window of 3 days 16½ hours, 3.7 days. The December window is a little shorter, just over 3 days 11 hours, nearly 3.5 days.

Venus Orbital Data

The Sidereal Period of Venus, the time it takes to orbit the Sun = 224.6987 days. 13 Orbits of Venus = 2921.083 days.

The Earth, of course, orbits the Sun once a year, 365.25 days. 8 Earth years = 2922 days, about 22 hours longer than 13 orbits of Venus.

The Synodic Period of Venus is the average time from one inferior conjunction to the next = 583.9214 days. This is 1.6 Earth years, also the time for 2.6 orbits of Venus, precisely 1 more. During 8 years, Venus overtakes the Earth 13 - 8 = 5 times. 5 Synodic Periods = 2919.607 days, 2.4 days less than 8 years.

During 8 years, five inferior conjunctions of Venus occur at positions regularly spaced round the orbits, about 72° apart.



The diagram shows the dates and positions of Venus when it is at inferior conjunctions from the transit in 2004 to the transit in 2012. At conjunctions the Earth, Venus and Sun are in line. The positions are indicated by the "spokes", red for the two transits. In small type the position of conjunctions up to 2020 are also marked.

Earth and Venus both orbit round the Sun in an anticlockwise direction, as seen looking down on the solar system from above the north pole. The 5 inferior conjunctions are evenly spread out round the orbits, but on the return to the transit node in 2012 the position is slightly before the 2004 position.

The difference represents 2.4 days, which is less than the length of the June window of opportunity, hence two transits separated by 8 years may occur, as in 2004 and 2012.

Circumstances in 2004, 2012 and 2020?

2004

   Earth at longitude of Venus node:   June 7  03:10:30 UT
   Venus at node:                      June 7  14:54:06 UT
   Mid transit:                        June 8  08:19:44 UT
  

Venus reaches the node nearly 11.75 hours after the Earth. It takes another 17.5 hours to catch up with the Earth when the transit occurs. That is some 29 hours after the Earth was at the node.

2012

   Venus at node:                       June 6  16:55:40 UT
   Earth at longitude of Venus node:    June 7  03:28:30 UT
   Mid transit:                         June 6  01:29:28 UT
  

Venus is at the node just over 10.5 hours before the Earth. It will have overtaken the Earth, when the transit occurs, almost 26 hours before the Earth reaches the node. The transit is 55 hours earlier than 2004.

2020? Venus reaches the node June 5 at 19:00. Inferior conjunction will be two days earlier, too early for a transit to occur.



The diagram shows the path of Venus relative to the Sun in 2004, 2012 and 2020. As can be seen in 2020 Venus will pass north of the Sun, so there is no transit.

Future and past transits of Venus

The small difference in the length of 8 Earth years and 13 years on for Venus means the positions of the inferior conjunction gradually rotate in a clockwise direction round their orbits. (This is the opposite direction to the planets in their orbits.) So after 8 years the "repeat" of the inferior conjunction on 2006 January 13 will occur slightly to its right in 2014 - on January 11.

By the year 2117 the position of the 2006 conjunction will have rotated to be almost at the ascending node, (shown by omega, Ω). This will result in an ascending node transit in 2117, 105½ years after the 2012 transit. Eight years later, in December 2125 the conjunction will be just the other side of the node and a further transit will occur.

The previous pair of transits were in December 1874 and 1882, the latter being 121½ years before the 2004 transit. Thus in all there is a cycle 243 years long during which 4 transits occur. 395 orbits of Venus take just a few hours more than 243 Earth years. Venus overtake the Earth 152 times in the same period.

Diagram adapted from Dave Herald's OCCULT program

The diagram shows the paths of the two transits of Venus in the 22nd century. The hemisphere of the Earth from which the start and end of each transit is visible is also shown.

Two cycles of transits

Diagram adapted from Dave Herald's OCCULT program

The diagram shows the path of Venus across the Sun at 8 transits from 1761 to 2125. The transit in 2012 is the first to have similar circumstances to the one which brought Cook to the south seas to observe in June 1769.

Even the 243 cycle is not exact. Each 243 years the paths of Venus across the Sun gradually move to the south. The effect can be seen in the diagram above, especially for the December transits. The result is that eventually as the path of the more northerly transit approaches the centre of the Sun, the southerly one moves off the Sun so that only single transits occur. The start and end of the current period of paired transits can be seen in the table below.

Dates of Transits of Venus, Year 0 - 4000 AD

 
   Descending node transits          Ascending node transits
  year mon day   year mon day       year mon day   year mon day 
    60 May 22                                       181 Nov 22
   303 May 24                                       424 Nov 22  
   546 May 24     554 May 22PS                      667 Nov 23  
   789 May 24     797 May 22                        910 Nov 23
  1032 May 24    1040 May 22                       1153 Nov 23
  1275 May 25    1283 May 23                       1396 Nov 23
  1518 May 25    1526 May 23        1631 Dec 07PS  1639 Dec 04
  1761 Jun 06    1769 Jun 03        1874 Dec 09    1882 Dec 06
  2004 Jun 08    2012 Jun 05        2117 Dec 10    2125 Dec 08
  2247 Jun 11    2255 Jun 09        2360 Dec 12    2368 Dec 10
  2490 Jun 12    2498 Jun 10        2603 Dec 15    2611 Dec 13PN
  2733 Jun 15    2741 Jun 13        2846 Dec 16    2854 Dec 14GS
  2976 Jun 16    2984 Jun 14       
                                    3089 Dec 18
  3219 Jun 19    3227 Jun 17        3332 Dec 20
  3462 Jun 21G   3470 Jun 19        3575 Dec 23
  3705 Jun 24GS  3713 Jun 21        3818 Dec 25
                 3956 Jun 23
  P = Partial,  G = Grazing
 

Data adapted from predictions given by Dave Herald's OCCULT program

As can be seen 8 year pairs of ascending node transits (November and December) only occur 6 times, from 1631 to 2854. Otherwise only single transits occur. The paired transits at the ascending node (May and June) occur over a much larger period from 546 to 3705, 14 pairs.

At a partial (P) transit, only part of Venus's disk will cross the Sun as seen from either the Northern or Southern parts of the Earth. Elsewhere a full transit will occur very close to the limb of the Sun.
At a grazing (G) transit, Venus's disk will graze the Sun, not move fully onto it. If followed by S the graze is only visible from southern parts of the Earth, from further north Venus will miss the Sun completely.

In later millennia still, after a period of single transits, the 8 year pairs will occur again.

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Transits of Mercury

Transits of Mercury are more common than those of Venus. The synodic period of Mercury is 115.88 days, less than one-fifth that of Venus. Thus there are 5 times as many chances for a transit to occur. Also the planet orbits closer to the Sun which also makes transit more likely. In the 21st century there are 14 transits of Mercury.

Like Venus, Mercury's transits occur at two well defined times of the year, in November from the 7th to the 14th or in May, between the 7th and 10th. November transits are more frequent, when Mercury is closer to the Sun than in May. Transits in November mostly occur at 13 year intervals, less frequently 7 year apart. The intervals between May transits normally alternate between 13 and 33 years.

In the current century November transits occur in 2006, 19, 32, 39, 52, 65, 78, 85 and 2098. May transits are in 2003, 16, 49, 62 and 2095 and are all followed 3½ years later by a November transit.

The diameter of Mercury is only 40% that of Venus, and it is about twice as far away. So it looks much smaller. At a transit Mercury's disk is too small to see by the unaided eye. A small telescope is required to make it visible as a black dot.

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