Introduction

The transit or passage of a planet across the disk of the Sun may be thought of as a special kind of eclipse. As seen from Earth, only transits of the inner planets Mercury and Venus are possible. Planetary transits are far more rare than eclipses of the Sun by the Moon. On the average, there are 13 transits of Mercury each century. In comparison, transits of Venus usually occur in pairs with eight years separating the two events. However, more than a century elapses between each transit pair.

The first transit ever observed was of the planet Mercury in 1631 by the French astronomer Gassendi. A transit of Venus occurred just one month later but Gassendi's attempt to observe it failed because the transit was not visible from Europe. In 1639, Jerimiah Horrocks and William Crabtree became the first to witness a transit of Venus.

In 1716, Edmond Halley published a paper describing exactly how transits could be used to measure the Sun's distance, thereby establishing the absolute scale of the solar system from Kepler's third law. Unfortunately, his method proved impractical since contact timings of the required accuracy are difficult to make due to the turbulence in Earth's atmosphere. Nevertheless, the 1761 and 1769 expeditions to observe the transits of Venus gave astronomers their first good value for the Sun's distance.

Photograph of the Transit of Venus on 1882 Dec 06.
Taken by students at Vassar College (Sky & Telescope Feb. 1961).

Transits of Mercury

At the present time, all transits of Mercury fall within several days of May 08 and November 10. Since Mercury's orbit is inclined seven degrees to Earth's, it intersects the ecliptic at two points or nodes which cross the Sun each year on those dates. If Mercury passes through inferior conjunction at that time, a transit will occur. During November transits, Mercury is near perihelion and exhibits a disk only 10 arc-seconds in diameter. By comparison, the planet is near aphelion during May transits and appears 12 arc-seconds across. However, the probability of a May transit is smaller by a factor of almost two. Mercury's slower orbital motion at aphelion makes it less likely to cross the node during the critical period. November transits recur at intervals of 7, 13, or 33 years while May transits recur only over the latter two intervals. Table 1 lists all transits of Mercury from 2001 through 2100. For a more complete and detailed list, see Seven Century Catalog of Mercury Transits: 1600 CE to 2300 CE.

* Separation is the angular distance (arc-seconds) between
the centers of the Sun and Mercury at mid-transit

To determine whether a transit of Mercury is visible from a specific geographic location, it is simply a matter of calculating the Sun's altitude and azimuth during each phase of the transit using information tabulated in the Seven Century Catalog of Mercury Transit. For the relevant equations and a sample calculation, see Transit Visibility. This web page also has links to several Excel files which perform the calculations automatically when the user inputs the latitude and longitude of any location.

Transits of Venus

Because Venus's orbit is considerably larger than Mercury's orbit, transits of Venus are much rarer. Indeed, only eight such events have occurred since the invention of the telescope (1631, 1639, 1761, 1769, 1874, 1882, 2004 and 2012). Transits of Venus are only possible during early December and June when Venus's orbital nodes pass across the Sun. Transits of Venus show a clear pattern of recurrence at intervals of 8, 121.5, 8 and 105.5 years. Table 2 lists all transits of Venus during the 800 year period from 1601 through 2400. For a more complete and detailed list, see Six Millennium Catalog of Venus Transits: 2000 BCE to 4000 CE.

* Separation is the angular distance (arc-seconds) between
the centers of the Sun and Venus at mid-transit

To determine whether a transit of Venus is visible from a specific geographic location, it is simply a matter of calculating the Sun's altitude and azimuth during each phase of the transit using information tabulated in the Six Millennium Catalog of Venus Transits. For the relevant equations and a sample calculation, see Transit Visibility. This web page also has links to several Excel files which perform the calculations automatically when the user inputs the latitude and longitude of any location.

The 2004 transit of Venus was visible from Europe, Africa and Asia. However, the final stages of the event were also visible from the eastern USA and Canada. Complete details can be found at 2004 Transit of Venus.

The 2012 transit of Venus will be visible from North America, the Pacific, Asia, Australia, eastern Europe, and eastern Africa. Details can be found at 2012 Transit of Venus.

Transit Predictions

• 1999 Nov 15: Transit of Mercury
• 2003 May 07: Transit of Mercury
• 2004 Transit of Venus (Observer's Handbook 2004)
• 2004 and 2012 Transits of Venus
• Predictions for the 2004 Transit of Venus
• 2006 Transit of Mercury (Observer's Handbook 2006)
• 2012 Transit of Venus (Observer's Handbook 2012)
• 2016 Transit of Mercury (Observer's Handbook 2016)
• 2019 Transit of Mercury (Observer's Handbook 2019)

• Seven Century Catalog of Mercury Transits: 1600 CE to 2300 CE
• Six Millennium Catalog of Venus Transits: 2000 BCE to 4000 CE

Acknowledgments

Transit predictions were generated on a Macintosh G4 computer using algorithms developed from the Explanatory Supplement [1974] and Meeus [1989]. Ephemerides for the Sun, Mercury and Venus were generated from JPL DE430 and DE431.

All calculations and diagrams presented in here are those of the author and he assumes full responsibility for their accuracy. Permission is freely granted to reproduce this data when accompanied by an acknowledgment:

"Transit Predictions by Fred Espenak, EclipseWise.com"

References (Transit Predictions)

Espenak, F., 2018, Six Millennium Canon of Transits: -1999 to +4000, Astropixels Publishing, Portal, AZ.

Explanatory Supplement to the Astronomical Ephemeris and the American Ephemeris and Nautical Almanac, 1974, Her Majesty's Nautical Almanac Office, London.

Macdonald , P., 2002, "The transit of Venus on 2004 June 8", J. Brit. Astr. Assoc., 112, 6, pp 319-324.

Meeus, J., 1956,"Transits of Mercury, 1920 to 2080", J.B.A.A., 67, 30.

Meeus, J., 1958,"Transits of Venus, 3000 BC to AD 3000", J.B.A.A., 68, 98.

Meeus, J., 1989, Transits, Willmann-Bell, Inc., Richmond.

Newcomb, S., 1895,"Tables of the Motion of the Earth on its Axis Around the Sun", Astron. Papers Amer. Eph., Vol. 6, Part I.

Newcomb, S., 1898,"Transits of Mercury, 1677-1881", Astron. Papers Amer. Eph., Vol. 6, Part IV.

References (History of Transits)

Berry, Arthur. "A Short History of Astronomy" (1898). Good general information. Includes detailed description of 1761-69 transits.

Chapman, Allan. "Jeremiah Horrocks, the Transit of Venus, and the 'New Astronomy' in early seventeenth-century England". Qtrly. J. Royal Astr. Soc, 31 (1990) pp 333-357. An appraisal of Horrocks' achievement; an attempt to dispel some myths which surround him, and a discussion of his methods.

Ferris, Timothy "Coming of Age in the Milky Way", esp. pp 130-135. A very readable account of 17th century attempts to use the transit of Venus to measure the solar parallax.

Gaythorpe, S.B. "Horrocks Observations of the Transit of Venus 1639 November 24 (O.S.)". J. Brit. Astr. Assoc., 47 (1936-7) pp 60-68. This paper gives a detailed quantitative account of Horrocks' observations and the circumstances in which they were made.

Halley, Edmond. "A new Method of determining the Parallax of the Sun" (Phil.Trans., Royal Soc., Vol xxix, 1716 pp 454-464).

Hetherington, Barry. "An Astronomical Anniversary: The Transit of Venus 1769 June 3". J. Brit. Astr. Assoc., 80 (1969) pp 52-53. A short summary of various expeditions.

Horrocks, Jeremiah. "Venus in sole visa" (1662) translated as "The Transit of Venus over the Sun" and published in "Memoir of the Life and Labors of the Rev.Jeremiah Horrox" by Rev.A.B.Whatton (London, 1859)

Maor, Eli. "June 8, 2004 - Venus In Transit", (1999) Princeton University Press. An excellent account of the history of transit expeditions and their historical importance to astronomy.

Maunder, Michael and Moore, Patrick . "Transit: When Planets Cross the Sun", (1999) Springer-Verlag. Another excellent account of the history of transit expeditions with details not covered in Maors' book. Includes practical information on observing transits.

Newcomb, S., 1898,"Transits of Mercury, 1677-1881", Astron. Papers Amer. Eph., Vol. 6, Part IV.

Pannekoek, Anton. "A History of Astronomy" (1961) Good general information. Includes detailed description of 1761-69 transits.

Porter, J.G. "Transits of Mercury and Venus". J. Brit. Astr. Assoc., 80 (1970) pp 183-189. A very useful discussion of the theory of transits, with some reference to Halley's method of determining the solar parallax.

Ruddy, H.E. "The Transit of Venus, 1874". J. Brit. Astr. Assoc., 64 (1954) pp 304-309. A diary style account of the expedition of Lt. C.Corbet to Kerguelen Island.

Westfall, Richard S. "Jeremiah Horrocks" + "Edmond Halley". Internet, Galileo Project. Key facts relating to these two astronomers. Useful bibliographies.