A Halley-type comet that goes round the Sun every 134 years, at perihelion in December 1992
Last at perihelion in December 1992. Next return about 2126.
Last at perihelion in December 1992. Next return about 2126.
Tonight from Georgetown it is highest at 4:30 am GYT, 27 degrees up in the south-east (a third of the way up). When the sky turns fully dark at 6:50 pm it stands -63 degrees up in the south-west. It is at least 10 degrees up for 1.2 hours of darkness.
At about magnitude 37.2 (forecast) it reads 37.6 through the air at that height. From Georgetown's sky, where the faintest star overhead is about magnitude 4.3, a comet has to read 3.3 for the naked eye, 6.8 for binoculars, 8.4 for a small telescope and 12.1 for a camera. So it is out of reach tonight.
From Lethem, under a dark interior sky: Last at perihelion in December 1992. Next return about 2126.
Face south-east (azimuth 115 degrees) at 4:30 am GYT and look a third of the way up, 27 degrees above the horizon. A fist held at arm's length covers about 10 degrees.
A comet's gas tail points away from the Sun: at that minute the anti-Sun direction leans 26 degrees to the right of straight up. It is 49 degrees from the Sun in the sky and 37 degrees from the Moon, which is 3% lit.
Against the stars it moves about 3 arcseconds an hour, so it is in the same field of view all night and a little further along the next night.
Nobody has reported its brightness to COBS in the last 30 days, so every number here is a forecast.
No one has followed this comet through a return like this one, so the forecast is a catalogue estimate with a wide range.
Catalogue estimates, shown as a cross-check and never as how bright it will look: JPL's law gives 37.2 tonight. Catalogue laws missed this season's measured comets by 2 to 4.5 magnitudes.
A comet's light is spread over a fuzzy patch, so it is harder to see than a star of the same magnitude. That is why the gear rungs ask for a magnitude in hand.
On a fixed tripod keep each exposure short enough that the stars stay round, then stack many frames. For a long stack, align on the comet as well as on the stars: it drifts against them from frame to frame. A dark site adds more than any lens.
109P/Swift-Tuttle passed closest to the Sun (perihelion) on 17 December 1992, at 0.959 au, where 1 au is the Earth's distance from the Sun. One orbit takes 134 years. The orbit is tilted 112.9 degrees to the Earth's and its eccentricity is 0.9633 (0 is a circle, 1 an open curve).
Tonight it is 43.03 au from the Sun and 43.68 au from Earth, moving away from the Sun.
Orbit: NASA/JPL Horizons osculating elements, solution JPL#32, retrieved 8 October 2026. Positions on this page are computed from them in your browser and agree with JPL's own tables to half an arcminute.
The parent of the Perseid meteors of August. It last passed the Sun in 1992 and will not return until 2126.
109P/Swift-Tuttle is a Halley-type comet that goes round the Sun every 134 years. The "P" in 109P marks a periodic comet, and the number is the order in which its return was first confirmed.
Its dust is the source of the Perseids meteors: each year the Earth crosses the trail it has left along its orbit.
Orbits: NASA/JPL Horizons (Giorgini, JD and the JPL Solar System Dynamics Group), data retrieved 2026-10-08.
Catalogue brightness laws: JPL Small-Body Database and the IAU Minor Planet Center.
Observed brightness: COBS Comet Observation Database (https://cobs.si) and its contributing observers, CC BY-NC-SA 4.0.
Data updated 8 October 2026. "Early forecast" means: A catalogue estimate. Early predictions are often wrong in both directions.
Gear rungs and ranges are estimates for a typical observer: your eyes, your sky and the night will differ.