The Comet Finder shows you which comets you can see tonight from where you are, where to look for each one and roughly how bright it should be. It reads today’s orbits of about a thousand comets from the Minor Planet Center, works out where each one sits as seen from your location, and lists the ones that are bright enough and climb high enough while the sky is dark.
How to use it
- Start with your location. You can let the browser read it, or type your latitude and longitude yourself. Either way, it stays in your browser for the visit and is never stored.
- Next, pick the time. The Comet Finder follows the clock until you tell it otherwise. To plan ahead, choose another date and time, or jump to tonight to see which comets climb above your lowest altitude while the sky is dark.
- Set the faintest magnitude you can see. That is about 6 for the naked eye, about 9 for binoculars and about 12 for a small telescope. Any comet fainter than your limit drops off the list.
- Set your lowest altitude as well. A comet that never gets above it during the dark window is left out, so raise it if trees, roofs or haze block the sky near your horizon.
- Now read the all-sky chart. It draws the sky above you as a disc, with the zenith in the centre, north at the top and east on the left. That is the sky as you see it lying on your back with your head to the north. You can zoom in up to 8 times and drag to pan around.
- Select a comet on the chart or in the table, then open its finder chart. The comet sits in the middle among the stars, with its track drawn over a month, so you can star-hop your way to it.
- Finally, choose how much sky the finder chart covers: 10°, 20°, 40° or 80° across its shorter side. A wide span makes it easier to find your way from the bright stars, while a narrow one shows exactly where the comet sits among the faint ones.
How it works
Every comet starts from the orbit the Minor Planet Center publishes for it, which gives the perihelion distance and date, the eccentricity, three angles, and the two brightness numbers H and G. The Comet Finder runs that orbit forward as a two-body problem, meaning the comet moves under the Sun’s gravity alone. That gives its distance from the Sun and from Earth at the moment you chose. The position is then turned into an altitude and a direction for your location, and it comes out good to well under an arcminute.
A comet counts as up now when the sky is dark and it stands above your lowest altitude. It is up tonight if that happens at any point in the dark window, which is the first unbroken stretch in the next 24 hours with the Sun more than 12° below the horizon.
The predicted magnitude comes from the formula below. It assumes a comet that brightens steadily as it closes in on the Sun, and real comets don’t always behave that way. An active one can turn out a magnitude or more brighter or fainter than predicted. Use the number to decide what is worth trying, and don’t take it as a promise.
Formula
m = H + 5 · log10(Δ) + 2.5 · G · log10(r)
Here m is the predicted total magnitude. H is the comet’s absolute magnitude and G its slope, and both come from the Minor Planet Center. Δ is the comet’s distance from Earth and r its distance from the Sun, both measured in astronomical units (au).
Worked example
Take H = 7.0, G = 4.0, Δ = 1.2 au and r = 1.5 au. Then m = 7.0 + 5 · log10(1.2) + 2.5 · 4.0 · log10(1.5) = 7.0 + 0.4 + 1.8 = 9.2. A comet like that is within reach of binoculars, but you won’t see it with the naked eye.
Common questions
How do I know if a comet is bright enough to see?
Compare its predicted magnitude with the faintest magnitude your instrument can reach, which is about 6 for the naked eye, 9 for binoculars and 12 for a small telescope. A dark sky matters more than usual here. A comet is a large, dim smudge rather than a point, so a bright sky washes it out well before it would hide a star of the same magnitude.
Why does the Comet Finder show no comets?
There are two possible reasons. Either no comet is both brighter than your faintest magnitude and higher than your lowest altitude during the dark window, or the orbits could not be read. In the first case, raise the faintest magnitude, lower the lowest altitude or try another night. In the second, the Comet Finder tells you the Minor Planet Center can’t be reached and offers to read again.
How accurate are the positions and the magnitudes?
The positions are good to well under an arcminute, far tighter than any finder chart needs. Brightness is the weak spot. The magnitude is only a prediction, and an active comet can come in a magnitude or more brighter or fainter.
Is my location stored?
No. There is no account to sign in to, and your location stays in your browser for the length of your visit. It is never sent to us and never kept.
Can I plan for a place other than where I am?
Yes. Type the latitude and longitude of any site you like, then the date and time you expect to be there. Until you set a location, the Comet Finder works from the fixed observing site that the status line on the home page reports.
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