Mercury and Venus transits across the Sun

Why every inferior conjunction does not produce a transit

Mercury passes between Earth and the Sun several times a year; Venus does so about every 584 days. Usually, however, each passes north or south of the apparent solar disk. Their orbits are tilted to the ecliptic by about seven degrees for Mercury and 3.4° for Venus, much larger than the Sun’s apparent half-degree diameter.

A transit needs inferior conjunction close to a node, where the orbital plane crosses Earth’s orbital plane. Earth looks along those node directions only during particular parts of the year: Mercury transits occur in May or November, Venus transits in June or December under the familiar present-era pattern.

Conjunction is therefore necessary but not sufficient. A planet close to the Sun in a sky map is not automatically crossing its disk, and a transit’s global existence does not mean the Sun is above every observer’s horizon during it.

Mercury transits in May and November

Mercury transits occur about thirteen or fourteen times a century. November events are more frequent than May events because Mercury’s eccentric orbit changes the node-crossing geometry and the window that allows a transit.

The apparent planetary disk is tiny, around ten to twelve arcseconds against the Sun’s roughly nineteen-hundred-arcsecond diameter. It cannot be resolved with the unaided eye through eclipse glasses. A suitable safely filtered telescope and adequate magnification are needed; around fifty times magnification is a useful reference, with visibility also depending on the instrument and conditions.

A transit can last several hours. Mercury’s round, dark disk moves progressively across the solar face, unlike the irregular shapes of sunspots and their surrounding penumbrae. Whether you can follow the whole event depends on its contacts relative to local sunrise and sunset, plus weather and horizon obstructions.

For an observing session at the Grand Canyon or in New York, check the local contact times and the Sun’s altitude. The global orbital event is shared, but the available sunlight and contact visibility are local.

Venus transits: pairs separated by long gaps

Venus transits are much rarer. A familiar long-term pattern repeats over about 243 years: pairs eight years apart, separated by gaps of roughly 121.5 and 105.5 years. The previous pair occurred in 2004 and 2012. The next pair belongs to the twenty-second century under that pattern, rather than offering another event in the current century.

The eight-year pairing comes from a close orbital resonance: eight Earth years are nearly thirteen Venus orbits and five synodic periods. After eight years the alignment nearly repeats, with a small shift relative to the node. A second transit can occur, while another repetition then misses the disk.

Historically, Venus transits helped establish the scale of the solar system. Edmond Halley proposed comparing timings from distant observing sites to determine solar parallax and the astronomical unit. Expeditions followed for the 1761 and 1769 events; James Cook’s 1769 Tahiti observation is a documented historical example and stays in the account rather than being geographically relocated.

Venus’s larger apparent disk, close to an arcminute, can be detected with suitable unaided-eye solar protection under favourable conditions. Its atmosphere can also refract sunlight into a thin aureole near ingress or egress. This is different from assuming that any dark shape near the Sun is a transiting planet.

Contacts and the black-drop effect

Four contacts describe a non-grazing transit. At first contact, the planetary disk touches the solar limb from outside. At second contact, it has moved fully inside. Third and fourth contacts describe the reverse sequence at egress. Maximum transit lies between them, at the smallest separation from the solar centre.

A grazing geometry may lack internal contacts. The observer’s topocentric viewpoint changes the precise timings through parallax, so geocentric and local contacts need not match. Instrumental resolution and atmospheric turbulence can also make an observed contact less sharply defined than the mathematical disk tangency.

Near the internal contacts, the black-drop effect can make the planet appear briefly connected to the solar limb by a dark neck. Optical image formation, the solar limb’s brightness profile and atmospheric conditions contribute, complicating historical timing measurements. Numerical precision does not remove those observational limits.

Safe observation

Never look at the Sun through unfiltered binoculars, telescopes or an optical camera viewfinder. Solar filters must be designed for the instrument and securely fitted over the front of the optics. Eclipse glasses worn at the eyes do not make an unfiltered telescope safe, and an eyepiece filter exposed to concentrated sunlight is not an acceptable substitute.

A purpose-designed solar projection instrument offers another route when set up and supervised correctly. Do not assume ordinary optics can safely withstand projection use. Consult the relevant equipment instructions rather than improvising from a short article. The tiny size of Mercury is no reason to reduce solar protection.