Venus Transit of the Sun – What Is It, How Does It Happen, and Other Historical Transits of Venus

by Robert Wilkinson

A “Transit of Venus” is when it makes an Inferior Conjunction with the Sun while retrograde. Though Venus goes retrograde fairly frequently, it only makes a “transit of the Sun” twice every 105-121 years. How does this happen, and who on Earth gets to see this one?

For those who want to know the astronomy of the phenomenon, here’s a bit from Wikipedia:

A transit of Venus across the Sun takes place when the planet Venus passes directly between the Sun and Earth, becoming visible against (and hence obscuring a small portion of) the solar disk. During a transit, Venus can be seen from Earth as a small black disk moving across the face of the Sun. The duration of such transits is usually measured in hours (the transit of 2004 lasted six hours). A transit is similar to a solar eclipse by the Moon. While the diameter of Venus is almost four times that of the Moon, Venus appears smaller, and travels more slowly across the face of the Sun, because it is much farther away from Earth.

Transits of Venus are among the rarest of predictable astronomical phenomena. They occur in a pattern that repeats every 243 years, with pairs of transits eight years apart separated by long gaps of 121.5 years and 105.5 years. The periodicity is a reflection of the fact that the orbital periods of Earth and Venus are close to 8:13 and 243:395 commensurabilities.

The next transit of Venus will occur on 5 and 6 June 2012, and will be the last Venus transit this century; the prior transit took place on 8 June 2004. The previous pair of transits were in December 1874 and December 1882. After 2012, the next transits of Venus will be in December 2117 and December 2125.

Venus transits are historically of great scientific importance as they were used to gain the first realistic estimates of the size of the Solar System. Observations of the 1639 transit, combined with the principle of parallax, provided an estimate of the distance between the Sun and the Earth that was more accurate than any other up to that time. In addition, the June 2012 transit will provide scientists with a number of other research opportunities, particularly the refinement of techniques to be used in the search for exoplanets.

A transit of Venus can be safely observed by taking the same precautions used to observe the partial phases of a solar eclipse. Staring at the Sun without appropriate eye protection can quickly cause serious and often permanent eye damage.

Venus, with an orbit inclined by 3.4° relative to the Earth's, usually appears to pass under (or over) the Sun in the sky at inferior conjunction. A transit occurs when Venus reaches conjunction with the Sun at or near one of its nodes—the longitude where Venus passes through the Earth's orbital plane (the ecliptic). Although the inclination between these two orbital planes is only 3.4°, Venus can be as far as 9.6° from the Sun when viewed from the Earth at inferior conjunction. Since the angular diameter of the Sun is about half a degree, Venus may appear to pass above or below the Sun by more than 18 solar diameters during an ordinary conjunction.

Sequences of transits occur in a pattern that repeats every 243 years, with transits occurring eight years apart followed by a gap of 121.5 years, then a gap of eight years and then another long gap of 105.5 years. The pattern repeats every 243 years because 243 sidereal orbital periods of the Earth (365.25636 days—slightly longer than the tropical year) is 88757.3 days, and 395 sidereal orbital periods of Venus (224.701 days) is 88756.9 days. Thus, after this period both Venus and Earth have returned to very nearly the same point in each of their respective orbits. This period of time corresponds to 152 synodic periods of Venus.

The pattern of 105.5, 8, 121.5 and 8 years is not the only pattern that is possible within the 243-year cycle, because of the slight mismatch between the times when the Earth and Venus arrive at the point of conjunction. Prior to 1518, the pattern of transits was 8, 113.5 and 121.5 years, and the eight inter-transit gaps before the AD 546 transit were 121.5 years apart. The current pattern will continue until 2846, when it will be replaced by a pattern of 105.5, 129.5 and 8 years. Thus, the 243-year cycle is relatively stable, but the number of transits and their timing within the cycle will vary over time.

Here are previous Venus transits of the Sun, for those who want to construct charts of do more historical analysis. All times are UT.

1396 November 23 – Start 15:45, Middle 19:27, End 23:09 (Last transit not part of a pair.)
1518 May 25–26 Start 22:46 May 25, Middle 01:56 May 26, End 05:07 May 26
1526 May 23 Start 16:12, Middle 19:35, End 21:48 (Last transit before invention of telescope.)
1631 December 7 Start 03:51, Middle 05:19, End 06:47 (Predicted by Kepler)
1639 December 4 Start 14:57, Middle 18:25, End 21:54 (First transit observed by Horrocks and Crabtree)
1761 June 6 Start 02:02, Middle 05:19, End 08:37 (Lomonosov, Chappe d'Auteroche and others observe from Russia)
1769 June 3–4 Start 19:15 June 3, Middle 22:25 June 3, End 01:35 June 4 (Cook sent to Tahiti to observe the transit)
1874 December 9 Start 01:49, Middle 04:07, End 06:26 (Pietro Tacchini leads expedition to Muddapur, India. A French expedition goes to New Zealand's Campbell Island)
1882 December 6 Start 13:57, Middle 17:06, End 20:15 (John Phillip Sousa composes a march, "The Transit of Venus” in honor of the transit.)
2004 June 8 Start 05:13, Middle 08:20, End 11:26 (Various media networks globally broadcast live video of the Venus transit.)
2012 June 5–6 Start 22:09 June 5, Middle 01:29 June 6, End 04:49 June 6

This one is visible in its entirety from Hawaii, Alaska, Australia,New Zealand, the Pacific and Eastern Asia, with the beginning of the transit visible from North America and the end visible from Europe

Los Angeles - Start 3:09 pm June 5, Middle 6:29 pm June 5, End 9:49 pm June 5
New York - Start 6:09 pm June 5, Middle 9:29 pm June 5, End 0:49 am June 6
London - Start 11:09 pm June 5, Middle 2:29 am June 6, End 5:49 am June 6
New Delhi - Start 3:39 am June 6, Middle 6:59 am June 6, End 10:29 am June 6
Melbourne - Start 8:09 am June 6, Middle 11:29 am June 6, End 2:49 pm June 6

This page on the Transit of Venus is great if you live down under, since it gives drawings of when and where to look for this Venus transit if you’re in Adelaide, Melbourne, Sydney, and Auckland. The article also has several brief essays about other transits of Venus as well as this one, along with historical writings by some of the greats and notes about what was going on then.

In the next article in this series, we’ll explore more about the astrological implications of this “Inferior conjunction” between Venus and the Sun.

This post has links to all the previous Venus retrograde articles:

A Brief Look At the June 2012 Gemini-Sagittarius Lunar Eclipse and the Venus Retrograde Occultation Event

© Copyright 2012 Robert Wilkinson

Comments

[Jo]

What is the difference between June 4 which you use and June 5 which everyone else is saying?? I want to be in the right place at the right time to transmute the Exact Square that will occure 13 hrs later. Thanks Robert!!

[Robert]

Hi Jo - I just rechecked my work. Everywhere I looked I used June 5 for the Venus event. It's the Eclipse that happens on June 4. The Venus square Mars happens 13 hours after the Eclipse, and about 25 hours before the Venus event.

[Shehzaad Maroof Khan]

hi Robert we r sitting outside enjoying the full moon .... talking about venus transit of the sun grt article my brother! GOD bless you :))

[Matt]

Had a brilliant Transit of Venus experience this evening. It was a generally overcast afternoon in NY. After work, I had tickets to the ballet (thought it was appropriate that A Midsummer's Night Dream was being staged). I got to the theatre early - so I went in and upstairs and onto the outdoor balcony with 15-20 mins before the show began. Within minutes, the sun came out from behind the clouds and the balcony was bathed in rays right in the middle of the transit event (when visible locally). It felt like it was exactly the right place to be...