Astronomical transit
A transit is the astronomical event that occurs when a non-luminous body subtending a smaller angular diameter passes between an observer and another body which, though more distant, subtends a larger angular diameter.
The term is most frequently used to describe the motion of a planet beween a terrestrial observer and the Sun. This can happen only with inferior planets. To a pedant, an annular eclipse is a transit of the Moon.
In recent years the discovery of extra solar planetss has excited interest in the possibility of detecting their transits across their own stellar primaries. HD 209458b is the first such transiting planet to be discovered.
Transits of Venus with respect to Earth are rare; they occur in pairs 8 years apart, which are separated by more than a century. The last pairs were in 1874 and 1882; the next pairs will occur on June 8, 2004 and June 6, 2012; followed by 2117 and 2125.
In the 18th century, the transit pair of 1761 and 1769 was used to try to determine the value of the astronomical unit. Unfortunately, Venus's thick atmosphere made it impossible to time the exact moment of the start and end of the transit, making the calculations inexact.
Transits of Mercury with respect to Earth, being closer to the Sun, are more frequent, with about thirteen per century. They do not happen in pairs the same way as transits of Venus. The last ones were on November 15, 1999 and May 7, 2003; the next transits will occur on November 8, 2006 and May 9, 2016.
| Date of mid-transit | Time (UTC) | ||
|---|---|---|---|
| Start | Mid | End | |
| 1761 June 6>Jun 06 | 02:02 | 05:19 | 08:37 |
| 1769 June 3>Jun 03 | 19:15 | 22:25 | 01:35 |
| 1874 December 9>Dec 09 | 01:49 | 04:07 | 06:26 |
| 1882 December 6>Dec 06 | 13:57 | 17:06 | 20:15 |
| 2004 June 8>Jun 08 | 05:13 | 08:20 | 11:26 |
| 2012 June 6>Jun 06 | 22:09 | 01:29 | 04:49 |
| Dec 11 | 23:58 | 02:48 | 05:38 |
| Dec 08 | 13:15 | 16:01 | 18:48 |
| Jun 11 | 08:42 | 11:33 | 14:25 |
| Jun 09 | 01:08 | 04:38 | 08:08 |
| Dec 13 | 22:32 | 01:44 | 04:56 |
| Dec 10 | 12:29 | 14:45 | 17:01 |
| Jun 12 | 11:39 | 14:17 | 16:55 |
| Jun 10 | 03:48 | 07:25 | 11:02 |
| Date of mid-transit | Time (UTC) | ||
|---|---|---|---|
| Start | Mid | End | |
| 1970 May 9>May 09 | 04:19 | 08:16 | 12:13 |
| 1973 November 10>Nov 10 | 07:47 | 10:32 | 13:17 |
| 1986 November 13>Nov 13 | 01:43 | 04:07 | 06:31 |
| 1993 November 6>Nov 06 | 03:06 | 03:57 | 04:47 |
| 1999 November 15>Nov 15 | 21:15 | 21:41 | 22:07 |
| 2003 May 7>May 07 | 05:13 | 07:52 | 10:32 |
| 2006 November 8>Nov 08 | 19:12 | 21:41 | 00:10 |
| 2016 May 9>May 09 | 11:12 | 14:57 | 18:42 |
| 2019 November 11>Nov 11 | 12:35 | 15:20 | 18:04 |
| 2032 November 13>Nov 13 | 06:41 | 08:54 | 11:07 |
| 2039 November 7>Nov 07 | 07:17 | 08:46 | 10:15 |