Planetary aspects Transit
Transit
A small dark dot creeping across the Sun
An inner planet crosses the solar disc: rare, and only ever watched through a proper filter.
- Required alignment
- Inferior conjunction, close to the ecliptic plane
- Next Mercury transit
- 13 November 2032
- Last Venus transit
- 5–6 June 2012
01 / Overview
Meet Transit
When Mercury or Venus passes between Earth and the Sun close enough to the ecliptic plane, a small black disc creeps across the Sun: a transit. Orbital inclination makes the alignment rare — Mercury transits some 13 to 14 times a century, while Venus transits arrive in pairs eight years apart, separated by more than a century.
A yardstick for the solar system
Eighteenth-century astronomers travelled the world to observe transits of Venus, deriving the Earth–Sun distance from the difference in contact times recorded at different sites. It was the first reasonably precise measurement of the astronomical unit, and an early model of international scientific cooperation.
The same trick, applied to other stars
The same geometry drives the search for exoplanets: when a planet crosses a distant star, the star dims by a fraction of a percent. Kepler and TESS found thousands of planets by watching for exactly that periodic, tiny dip.
02 / Key data
Core parameters
- Required alignment
- Inferior conjunction, close to the ecliptic plane
- Next Mercury transit
- 13 November 2032
- Last Venus transit
- 5–6 June 2012
- Next Venus transit
- December 2117
- Recurrence
- About 13 Mercury transits per century
Data and calculation notes
Active periods, peak dates and rates are multi-year averages; each year differs a little, so plan against the forecast issued for the year in question.
03 / Observing tips
How to watch
- A certified solar filter is mandatory: never look directly, or through unfiltered optics.
- Projection is safe and lets a group watch together.
- Mercury’s disc is tiny: a telescope is needed, and eclipse glasses alone will not show it.
04 / Sources