Object Altitude Calculator: When Is It Highest Tonight?

Every star, cluster and galaxy crosses your sky once a day, and it looks its best at one moment: when it is due south and as high as it gets. Pick a target, your town and a date, and this works out when it rises, when it peaks, how high it climbs and where to point for every hour of the night. Height matters more than most beginners expect: at 30 degrees up you are looking through twice as much air as straight overhead, and at 10 degrees nearly six times as much.

Times are UK clock time, so GMT in winter and BST in summer, worked out for the night that starts on the date you pick. Coordinates are standard J2000 positions moved forward to your date for precession, rise and set allow for the usual half a degree of atmospheric refraction, and the table assumes a flat horizon. A row of houses or trees to the south can easily hide the first 15 degrees, so check the altitude column against what you can actually see from your garden.

How to read the result

The peak, called the transit or culmination, is the moment the target crosses the meridian, the imaginary line from due north through the point overhead to due south. For anything south of the point overhead, that is when it sits due south. Its height at that moment is fixed by simple geometry: 90 degrees minus the difference between your latitude and the target's declination. That is why the Orion Nebula, at about 5 degrees south of the celestial equator, peaks at roughly 33 degrees from London but only about 27 degrees from Inverness.

The calculator also checks the Sun. The "Sky" column marks each hour as daylight, twilight or dark (Sun more than 12 degrees down), and the best viewing time is the highest point the target reaches while the sky is dark. If the transit falls in daylight, the target is out of season, and you are better waiting a few months for it to peak at night. Each target transits about four minutes earlier every night, which adds up to two hours a month.

What altitude is good enough?

• Above 40 degrees: the best views. Less air means steadier, sharper images and better contrast on faint galaxies.
• 25 to 40 degrees: fine for most targets, and as good as it gets for southern objects such as the Orion Nebula from the UK.
• 10 to 25 degrees: workable for bright clusters and stars, but expect shimmer and dimming, and light pollution is usually worst near the horizon. Sagittarius targets such as M8 and M22 never get much higher than this from Britain.
• Below 10 degrees: mostly lost in haze, buildings and town glow.

Planets and the Moon move against the stars, so they are not in the list: use our guide on how to see the planets and the moon phase calculator for those. To see exactly when proper darkness starts and ends for your town, try the astronomical darkness calculator, and once you know where to point, how to find objects with a telescope covers star hopping from there. For two popular winter and autumn targets in depth, see finding the Orion Nebula and seeing the Andromeda Galaxy.