☄ Crommelin 27P/Crommelin

How to follow comet Crommelin live

The panel above recomputes the position of Crommelin every second in your browser: its distance from the Sun and from Earth, its position in the sky (right ascension and declination), and a live countdown to the next perihelion. It runs on the same kind of engine observatories use, a Kepler solver applied to the JPL osculating orbital elements, so the numbers are not a static snapshot, they keep ticking.

Just below, the top-down map of the Solar System shows exactly where Crommelin is right now among the planets. You can fast-forward time with the day slider, zoom and pan, compare its distance to another body with a click, and press "Next event" to jump straight to perihelion. It is the most direct way to grasp the orbit of Crommelin with no math at all.

Comet fact sheet

About Crommelin

27P/Crommelin is one of the rare intermediate-period comets, with about 27.4 years between perihelion passages. That places it in a sparsely populated orbital range and means an entire human generation passes between consecutive returns.

Three independent astronomers recorded it in different decades without recognising the connection: Pons in 1818, Coggia in 1873 and Forbes in 1928. It was the orbital calculation by Andrew Crommelin of Greenwich Observatory that unified all three sightings and revealed a single returning comet, leading the IAU to name it in his honour. The 2011 return (the most recent documented perihelion) confirmed all predictions.

History and discovery

Jean-Louis Pons spotted the comet in 1818 from Marseille. In 1873, Jerome Coggia, also in Marseille, discovered a comet without realising the connection to Pons. In November 1928, South African astrophysicist Andrew D. Forbes recorded a new naked-eye comet. All three returns were documented, but no one had computed the full orbit.

Andrew Claude de la Cherois Crommelin, deputy director of the Royal Observatory Greenwich, performed the definitive calculation and showed that Pons (1818), Coggia (1873) and Forbes (1928) had each seen the same object at 27-year intervals. Confirmation came with the 1956 return, predicted by Crommelin before his death in 1939. The IAU honoured his work by naming the comet 27P/Crommelin. The 2011 return was the most recent; the next perihelion is expected on 27 May 2039.

Orbit and returns

27P/Crommelin is classified as a Halley-type comet (HTC), the group defined by orbital periods between 20 and 200 years. With perihelion at about 0.74 AU from the Sun (closer than Venus) and aphelion at roughly 17.6 AU (between Uranus and Neptune), the orbit is highly eccentric (e about 0.92) and steeply inclined to the ecliptic plane (about 29 degrees).

The last documented perihelion occurred in August 2011. The next return is expected on 27 May 2039, with a period of approximately 27.38 years (10,000 days). The steeply inclined orbit suggests the comet did not form in the original flat protoplanetary disk.

Nucleus and dynamic classification

27P belongs to the Halley-type comet (HTC) class, defined by periods between 20 and 200 years and a Tisserand parameter relative to Jupiter T_J below 2. With aphelion at about 17.6 AU, the comet spends most of its orbit in the cold region between Uranus and Neptune, where temperatures drop below -200 degrees Celsius and volatile gases remain frozen in the nucleus.

The nucleus is estimated at a few kilometres in size, but the absence of spacecraft visits prevents direct measurement. The high eccentricity (e about 0.92) suggests the comet may have originated in the inner Oort Cloud or been perturbed by Saturn and Jupiter over millions of years. Its 29-degree inclination distinguishes it from Jupiter-family comets, which typically have inclinations below 20 degrees.

How to observe

On favourable returns 27P/Crommelin can reach magnitude 8 to 10, as in 1984, making it visible in binoculars and small telescopes. On less favourable returns (such as 2011 at magnitude 11) a 150 mm or larger telescope is needed. The 27-year period means most people will have only one or two chances to observe it in a lifetime.

For the 2039 return (perihelion expected 27 May), the best brightness window will fall in April through July 2039. Following JPL Horizons ephemerides from about 2037 onward is recommended, when updated orbital calculations will be available at high precision.

• Next perihelion: 27 May 2039
• Expected magnitude: 9 to 11 (depends on exact geometry)
• Recommended aperture: 100 mm to 200 mm
• Orbital period: 27.38 years (~10,000 days)

Science and historical importance

The comet's historical significance lies in the role it played in the history of celestial mechanics: linking three separate apparitions as a single object was a triumph of pre-computer orbital calculation, comparable to Edmond Halley's work on the comet bearing his name. Crommelin's method required reconciling planetary perturbations accumulated over 110 years of observations.

Crommelin published the prediction of the 1928 return in 1930, nine years before he died in 1939, and did not live to see the definitive confirmation with the precise numerical data of the 1956 return. The comet is therefore an emblematic case of a scientific prediction confirmed posthumously, alongside the Halley comet itself. The 2011 return was covered by modern telescope campaigns that measured detailed light curves and gas production rates.

Facts worth knowing

• Crommelin published the 1928 return prediction in 1930 and died in 1939, nine years before the 1956 return definitively sealed his identification.
• The comet is one of the few named after an orbital calculator rather than the visual observer who found it.
• With aphelion at about 17.6 AU, 27P spends most of its orbit in the cold region between Uranus and Neptune, at temperatures below -200 degrees Celsius.
• Its steeply inclined orbit (29 degrees) points to an origin outside the original protoplanetary disk, probably the inner Oort Cloud or a reservoir of randomly oriented orbits.
• Perihelion at 0.74 AU brings it closer to the Sun than Venus, heating intensely for a few months every 27 years.
• The next perihelion (27 May 2039) will be seen for the first time by people born after 2011, the year of the last return.

Other comets

See the full comet catalogue.