What the tool shows
- Which comets are above your horizon tonight, when each is highest, and in which compass direction.
- A live all-sky chart for your location, with constellation lines and named stars.
- Finder charts at 25°, 10° and 5°, with the comet's dated track and the direction its tail points.
- Predicted brightness, distance from Earth and from the Sun, and angle from the Sun.
- A red night mode, for use at the eyepiece without spoiling dark adaptation.
How the brightness figure works, and where it fails
Magnitude measures brightness and runs backwards: lower numbers are brighter. Magnitude 6 is roughly the faintest star the unaided eye can see from a properly dark site; magnitude 13 is about a hundred thousand times fainter. Each step of 1 is a factor of about 2.5.
Every magnitude here comes from NASA's JPL Horizons service, which predicts it from the comet's orbit and its measured behaviour on previous visits. It is the whole comet added together, coma and all.
It is a model, not a measurement, and it cannot see an outburst coming. Comets erupt without warning. In 2026 comet 220P/McNaught erupted twice in three months, brightening roughly twenty thousand times and going from a telescopic smudge near magnitude 17 to a binocular object around magnitude 6 or 7 — bright enough to become NASA's Astronomy Picture of the Day. A prediction fitted to that comet's past behaviour showed nothing of the sort. When a comet erupts, the figure on this page can be wrong by many magnitudes, and it is always wrong in the same direction: the comet is brighter than we say, never fainter.
So before driving anywhere, check a current observing report. ALPO Comet News publishes a monthly summary of what observers are actually seeing, and COBS collects visual estimates as they are made. If observers say a comet is brighter than this page does, they are right and we are wrong.
Why we do not recommend a telescope size
An earlier version of this page named a telescope for each comet, worked out from the predicted magnitude and the published limiting magnitudes that telescope makers print on their spec sheets. We have taken that out.
The arithmetic was sound, but it was being fed a number that can be eight magnitudes wrong. Turning an unreliable prediction into a confident equipment recommendation gives it an authority it has not earned, and points it at somebody's wallet. A page that tells you a comet needs a sixteen-inch telescope on the same morning that it is sitting in binoculars on the front of APOD is not helping anyone.
What we show instead is the number itself, labelled as a prediction, with the position and timing information that NASA does know to sub-arcsecond accuracy. Judging what will show a given comet is a matter for your own sky, your own eyes and current observing reports — and if you would like a second opinion, ask us. We sell telescopes for a living and we would rather talk you into the right one than let a formula do it badly.
Why a comet is harder than a star of the same magnitude
A star is a point. All its light lands in one spot, and the eye is good at picking out points. A comet is a fuzzy patch several times the apparent width of Jupiter, and the same amount of light spread over that area is far harder to notice — like the difference between a torch beam and the same bulb behind frosted glass. A magnitude 8 star is obvious; a magnitude 8 comet can be a faint smudge you might scan straight past.
This also cuts the other way for anyone with a camera. Stacked exposures reach far fainter than the eye ever can, which is why comets well beyond visual range are routinely photographed with small smart telescopes from dark sites.
Why your sky matters
Light pollution sets a floor that no aperture can dig under. Aperture gathers more light from the comet, but it gathers more skyglow with it, so the contrast between the two barely improves. A comet lost in the glow from a city centre tends to stay lost however large the telescope. Driving to a darker site usually changes the answer more than a bigger instrument does.
Where the data comes from
- Positions, predicted brightness, and which comets are listed — NASA JPL Horizons and the NASA Small-Body Database, refreshed every night. Credit NASA/JPL-Caltech.
- Stars and constellation figures — the Hipparcos and Yale Bright Star catalogues, as prepared by the d3-celestial project, Copyright (c) 2015 Olaf Frohn, used under the BSD 3-Clause licence.
- Sun and Moon positions — the standard truncated series given in Jean Meeus, Astronomical Algorithms.
- Sky-darkness classes — John E. Bortle's dark-sky scale, published in Sky & Telescope in 2001.
- Designations and constellation names — the International Astronomical Union.
What this page does not know
- Whether a comet is in outburst right now. The single biggest gap, and the reason the brightness figure carries a warning. It needs observers reporting what they see, and that data is not openly available to us.
- How diffuse each comet is. A tightly condensed comet is far easier to spot than a large diffuse one of the same total magnitude, and nobody publishes that openly either.
- Cloud and seeing. That is the Astronomy Weather Forecast.
Glossary
- Magnitude
- How bright something looks. Lower is brighter, and each step of 1 is about two and a half times. The unaided eye reaches magnitude 6 under a dark sky, roughly 4 from a city.
- Coma
- The fuzzy cloud of gas and dust around a comet. It is what you actually see — the solid nucleus is only a few kilometres across and far too small to make out.
- Outburst
- A sudden unpredicted brightening when fresh material is exposed, sometimes by a factor of thousands. 220P/McNaught erupted twice in 2026, brightening about twenty thousand times. No forecast based on past behaviour can anticipate one.
- AU
- Astronomical unit: the distance from the Earth to the Sun, about 150 million kilometres.
- Perihelion
- The point in the orbit closest to the Sun. Comets are usually most active near it, though the best view from Earth can come weeks before or after.
- Angle from the Sun
- How far the comet appears from the Sun in our sky. Under about 30 degrees it is buried in twilight no matter how bright it is.
- Bortle
- A 1 to 9 scale for light pollution. 1 is a genuinely dark wilderness sky, 9 is inner city. It changes what you can see more than your telescope does.
- Altitude
- How high something is above the horizon, in degrees. Below about 20 degrees you are looking through a lot more air, and everything looks worse.
Common questions
Why does this page not tell me what telescope I need?
Because the brightness figure it would have to be based on is a prediction that cannot see an outburst coming, and can be wrong by many magnitudes. We would rather show you the number, tell you plainly how reliable it is, and let you judge — or ask us — than dress an unreliable input up as confident advice about what to buy.
The magnitude here disagrees with another site. Which is right?
Ours is NASA's prediction from the comet's orbit, refreshed nightly. Some other sites publish what people actually saw through a telescope last week. When a comet is behaving normally the two agree closely. When it erupts, real observations will show it and a prediction will not. If you see reports of a comet far brighter than we list, believe them.
Why does a magnitude 8 comet look like nothing through my telescope?
Because that brightness is smeared across a fuzzy patch rather than concentrated in a point. Turn the magnification down, not up — a wide, bright, low-power view shows a comet far better than a narrow high-power one.
Can a bigger telescope beat light pollution?
Only up to a point. Aperture gathers more light from the comet, but it gathers more skyglow with it, so the contrast between the two barely improves. A comet lost in the glow from a city centre stays lost however large the telescope. Driving to a darker site changes the answer far more than buying a bigger instrument does.
Can I photograph a comet that I cannot see?
Routinely. Stacked exposures reach far fainter than the eye, so comets well beyond visual range are captured with quite small instruments from dark sites. A camera on a tracking mount, or one of the current smart telescopes, will record comets that no eyepiece would show.
How accurate are the charts?
Very. Star positions are standard catalogue data and comet positions come from NASA to well under an arcsecond, updated nightly. You can star-hop with them, or type the coordinates straight into a GoTo handset. The predicted brightness is the uncertain part, not the position.
Which way does a comet's tail point?
Always directly away from the Sun, never along the direction of travel. Solar radiation and the solar wind push the dust and gas outward, so the tail is anti-solar regardless of which way the comet is moving. The chart shows that direction for each comet.
Is any of my data sent anywhere?
No. Comet positions are fetched from a nightly file and that request carries nothing about you. Your observing site is used only inside your own browser and is never transmitted.
About these figures
Comet brightness is unpredictable in principle, not merely in practice. Positions and timings on this page are reliable to a fraction of an arcsecond; the brightness is a forecast and should be treated as one. Treat every figure here as a guide and never as a promise.
© 2026 Ontario Telescope and Accessories Inc. The code, layout, artwork and written explanations on this page are ours, and may not be copied, rehosted or republished without permission. The astronomy underneath them is not ours and we claim nothing over it: sidereal time, coordinate conversion, solar and lunar position, Kepler's and Barker's equations and the comet magnitude law are all long-established standard work, credited above, and free for anyone to use.