There are usually several comets within reach of a modest telescope on any given night. The hard part is knowing which ones are within reach of your telescope, on which nights, and at what time to look. The synopses are written to answer exactly that.
Jump to: Reading a synopsis Detectability Aperture What we assume Latitude Dates and times Finder charts
Comet magnitudes alone are a poor guide to visibility. A magnitude 9 comet with a large, diffuse coma can be far harder to see than a magnitude 11 comet that is tightly condensed. These predictions use the SkyTools visibility model, which considers magnitude, coma diameter and degree of condensation from recent observations, together with sky brightness, the moon, the comet's altitude, and the aperture and magnification in use.
Each comet has one line per latitude. A line looks like this:
Perceptible in a 4-inch (10 cm) telescope from a dark site near midnight from the 1st/2nd through the 14th/15th, low in the south throughout. In a 6-inch (15 cm) telescope it is perceptible from the 1st/2nd through the 19th/20th.
Read in order, that says: how hard the comet is to see, the smallest instrument that reaches it, the sky it assumes, the part of the night to look, the nights it applies to, and where in the sky to point. The second sentence tells you what the next aperture up buys you, and appears only when it buys enough to be worth saying.
When a comet's visibility changes during the month the line follows it: easy from the 1st/2nd through the 5th/6th, then perceptible through the 21st/22nd. When it drops out and returns, the gap is stated rather than glossed over.
Some comets are simply out of reach from a given latitude, in which case the line reads Not perceptible. That means not perceptible in any of the instruments listed below, on any night of the month, from that latitude.
Every date on the main page is anchored to a detectability category. These describe how the comet looks in the eyepiece once you are pointed at it, not how hard it is to find:
| Category | What to expect |
|---|---|
| obvious | Unmistakable at a glance. |
| easy | Seen immediately and held without effort. |
| perceptible | Seen with reasonable confidence, but not necessarily at once. May need averted vision and a little patience. |
| difficult | At the edge. Experienced observers under good conditions will often still see it. |
| challenging | Beyond most observers at that aperture. |
Unless stated otherwise, the dates given are the nights the comet should be perceptible. That threshold is a judgement call, and it will not be everyone's. Many observers will still find a comet on the nights it is merely difficult. Others will want it to be easy before they can be confident of what they are seeing.
The synopses report the smallest instrument from a fixed ladder that reaches the comet:
naked eye, 7x50 binoculars, 4-inch (10 cm), 6-inch (15 cm), 8-inch (20 cm), 10-inch (25 cm), 12.5-inch (32 cm), 14-inch (36 cm), 16-inch (41 cm), 18-inch (46 cm), 24-inch (61 cm).
If your telescope falls between two of these, read the larger one. Each is modelled as a generic telescope of that aperture with a standard set of eyepieces of 50° apparent field, so that the model can choose a sensible magnification for each comet on each night. Comets vary enormously in size, and the right magnification for a large diffuse coma is nothing like the right magnification for a small condensed one. Your own eyepieces will differ, which is one reason to treat the aperture as a good starting point rather than a hard requirement.
A prediction for a specific observer at a specific site is a very different thing from a prediction for a whole hemisphere. To say anything useful here we have to assume a generic observer at a generic site, and your own circumstances will differ in at least a few of the following ways.
The important point is that these assumptions do not all matter equally, and they do not matter equally on every night. A bright comet low in evening twilight is barely affected by light pollution at all, because the sky is already bright. A faint comet high in a dark sky is affected enormously. Where a line says a comet is easy or obvious, the assumptions below are unlikely to change your night. Where it says perceptible or difficult, they may well.
| We assume | If your situation differs |
|---|---|
| Sky brightness | A dark rural site where the line says "from a dark site", otherwise a sky in which the faintest stars visible to the naked eye are about magnitude 6. That is the sort of sky you get away from the bright lights of downtown, perhaps at the edge of a city. We call it a country sky in the synopses for brevity. A brighter sky can cost you an aperture or two on a faint comet, and almost nothing on a bright one in twilight. |
| Transparency | Typical for the site, at sea level. A clear, dry night at altitude is worth real aperture: observers on a mountain will often do better than these predictions suggest, and observers under haze or thin cloud worse. |
| Experience | An intermediate observer. Newcomers should expect to see less than is predicted here. Highly experienced observers see more, and often go to considerable lengths to protect their night vision, such as wearing an eye patch before observing. Knowing what a faint comet actually looks like is itself worth a good deal. |
| Latitude | One of the four latitudes listed. The further you are from the one you are reading, the more approximate the dates and times become. |
| Equipment | A fixed ladder of apertures, each a generic telescope of that size with standard 50° eyepieces. |
The last date listed is the least certain. It is, by definition, the night the comet crosses the threshold, so it is where every one of the differences above lands. Expect it to move by a night in either direction, and sometimes more. The interior dates are much firmer than the endpoints.
None of this makes the predictions useless. Knowing that a comet is within reach of a 6-inch telescope in the second week of the month, best placed near midnight, is a great deal more than knowing its magnitude. But it is a starting point for planning a night, not a guarantee. For predictions matched to your own site, elevation, telescope, eyepieces and experience, use SkyTools.
Find the latitude closest to your own:
| Line | Use if you are in |
|---|---|
| 55°N | Great Britain, northern Europe, Russia, most of Canada |
| 40°N | The United States, southern Europe, China, Japan |
| Equator | Equatorial regions |
| 30°S | Australia, South Africa, southern South America |
If you are at an intermediate latitude, say 20°N, read the two nearest lines and expect something between them. A comet low in the sky is the most sensitive to this: a few degrees of latitude can add or remove several nights at the beginning or end of a run, and can change the time of night at which the comet is best placed.
All dates refer to the night, which is the date of the local evening. Anything before local noon belongs to the previous evening's date. The night of the 3rd means the evening of the 3rd and the morning of the 4th.
Because that trips people up, dates are written in the form that suits the time of night:
| Written as | Means |
|---|---|
| the 25th | The evening of the 25th. Used when the comet is best placed before midnight. |
| the 25th/26th | The night that begins on the evening of the 25th and ends on the morning of the 26th. Used when the best time is near midnight, or when it shifts across midnight during the month. |
| the 26th | Where the line says the comet is in the pre-dawn sky or in morning twilight, the date given is the morning date. Set the alarm for the small hours of the 26th. |
The part of the night to look is named directly. In order through the night: in evening twilight, in the evening, near midnight, after midnight, in the pre-dawn sky, in morning twilight. When the comet drifts from one to another over the course of the month, the line says so.
Where a line gives a position, altitude is described as very low (below 10°), low (10–20°), fairly high (20–40°) or high (above 40°). A compass direction is given only when the comet is below 30°, where knowing which way to face actually matters.
All times are local standard time. If daylight saving time or summer time is in effect where you are, add one hour.
Treat every time given here as approximate. It is computed for the latitude on that line, and your own latitude and longitude will shift it. It is also the time the comet is best placed, not the only time it can be seen: on most nights there is a usable window of an hour or more either side. Go out well before the time indicated and start looking. You will often pick the comet up early, and if you do not, you will at least be dark adapted and pointed at the right part of the sky when the good window opens.
The charts have north up and east to the left, like an atlas, unless indicated otherwise. Times and dates on the charts are UT. Several stars are labelled on each chart to help you locate the field; in some cases you may need to find the field in your atlas first. The charts are made with SkyTools.
Do the charts look grainy? Your browser is probably shrinking the chart to fit the window. Click the chart to zoom to full resolution.
A note about the charts. These are provided because people ask for them, and in the hope that they encourage beginners to try a telescopic comet or two. In practice the best charts are the ones you make yourself, set for your own location, date and time. Most astronomy software plots comet positions accurately as long as you keep the orbital elements current. What software rarely gets right is the size and appearance of the coma, which is what you actually need in order to recognise the comet when you find it. Use this page to work out which comets are within reach of your telescope and how large they will appear, then use your own software for the finder chart.
Symbols used: ° = degrees, ′ = arc-minutes, ″ = arc-seconds, ~ = approximately, ? = questionable, usually because there are no recent observations. A coma of ~4′ means the coma is approximately 4 arc-minutes across.