The comet 21P/Giacobini-Zinner
From Fairfield , 21P/Giacobini-Zinner is visible between 21:27 and 03:46. It will become accessible around 21:27, when it rises to an altitude of 21° above your south-eastern horizon. It will reach its highest point in the sky at 00:38, 37° above your southern horizon. It will become inaccessible around 03:46 when it sinks below 22° above your south-western horizon.
Object Type Comet
|Magnitude:||23.88 (V) |
|Absolute mag (H):||9.40 |
|Slope parameter (n):||5.84 |
|Right ascension:||11h52m |
39.77 lightmin 
|Semi-major axis:||3.50 AU|
|Longitude ascending node:||195.35°|
|Argument of perihelion:||172.85°|
|Epoch of elements:||08 March 2021|
|Mean Anomaly at epoch:||137.14°|
|Orbital period:||6.54 years|
All times shown in Fairfield local time.
|24 Mar 2025||– 21P/Giacobini-Zinner at perihelion|
The position of this comet was calculated from orbital elements published by the Minor Planet Center (MPC).
We estimate the brightnesses of comets from magnitude parameters published by the BAA Comet Section, where these are available. These are computed from the observations they receive from amateur astronomers.
Comets are intrinsically highly unpredictable objects, since their brightness depends on the scattering of sunlight from dust particles in the comet's coma and tail. This dust is continually streaming away from the comet's nucleus, and its density at any particular time is governed by the rate of sublimation of the ice in the comet's nucleus, as it is heated by the Sun's rays. It also depends on the amount of dust that is mixed in with that ice. This is very difficult to predict in advance, and can be highly variable even between successive apparitions of the same comet.
In consequence, while the future positions of comets are usually known with a high degree of confidence, their future brightnesses are not. For most comets, we do not publish any magnitude estimates at all. For the few comets where we do make estimates, we generally prefer the BAA's magnitude parameters to those published by the Minor Planet Center, since they are typically updated more often.