The comets you can look for this year: the ones within reach of binoculars or a small telescope, each a bar on the timelines of your cities over the weeks it is observable. Binocular comets have one colour, telescope comets another.
Select a comet for its dates, how bright it is expected to get and when, its perihelion - its closest point to the Sun - and its closest approach to the Earth, the hemisphere that sees it better and whether it is in the morning or the evening sky. Every card links to the forecast it comes from.
Comets are the hardest objects in the sky to predict. Their brightness depends on how much gas and dust they give off near the Sun, and a comet can outperform its forecast, fall short of it or break up. The calendar is kept up to date from published forecasts - Jonathan Shanklin's Comet Prospects for the British Astronomical Association and the yearly comet guides - and where they disagree, the card gives both.
Even a binocular comet is a faint, fuzzy patch: look from a dark site, away from city lights and with no Moon in the sky.
If a comet's bar crosses tonight on the timeline, it is within reach of the equipment named in its card - binoculars or a telescope - from a dark site. Select it for how bright it should be, the hemisphere and whether it is in the morning or the evening sky.
Only a few comets a decade get bright enough to see easily without optics, and most are found only weeks or months before. The comets on this calendar need binoculars or a telescope; a comet that becomes a naked-eye sight is added when it is forecast.
The next event button jumps to the next comet's window. Each bar covers the weeks the comet is within reach, and the card gives the dates of its perihelion - its closest point to the Sun - and of its closest approach to the Earth.
Less accurate than for anything else in the sky. A comet can brighten or fade well away from the forecast, or break up. The calendar follows published forecasts, and where they differ the card says so.
In Astronomy. Perfect Clock — a world clock with your calendar across time zones.