Tonight's Sky
What is up tonight where you are, which way to look, and when it gets dark.
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About the Tonight's Sky converter
Point of this page: you walk outside, see something bright, and want to know what it is. It tells you which planets are above your horizon right now, which compass direction to face and how high to look, what phase the Moon is in, and the times tonight actually gets dark.
Everything is worked out on your own device from orbital mechanics. There is no server, no account and no lookup — which also means it keeps working with the phone in aeroplane mode halfway up a hill, which is roughly where you want it.
It answers the two questions any stargazing session starts with — what is in the sky tonight, and when is it dark enough to bother — and then the ones that follow: moonrise and moonset, how much of the Moon is lit, and which of the planets up tonight are worth binoculars.
Is that bright thing a planet?
Almost always, yes. If something is noticeably brighter than everything around it and is not twinkling much, it is a planet. Venus is unmistakable — it is brighter than every star in the sky by a wide margin and sits low in the west after sunset or low in the east before dawn, which is why it has been called both the evening star and the morning star for as long as anyone has written things down. Jupiter is the next brightest, steady and cream-coloured. Mars is orange and varies enormously depending on how close it currently is.
The quickest test with your own eyes is twinkling. Stars are so far away that they are effectively points, and the atmosphere makes points flicker. Planets are close enough to show a tiny disc, and the flickering across that disc averages out — so a light that shines steadily is usually a planet, and one that sparkles is usually a star.
When it actually gets dark
Sunset is not darkness, and the gap between them is bigger than most people expect. There are three named stages afterwards. Civil twilight lasts until the Sun is 6° below the horizon and is when you can still comfortably read outdoors. Nautical twilight runs to 12° and is named for the point at which the sea horizon becomes too indistinct to take a sextant sighting against. Astronomical twilight ends at 18°, when the last scattered sunlight leaves the sky and faint objects finally become findable.
Above roughly 49° of latitude — which includes most of Britain, northern Europe and Canada — the Sun never reaches 18° below the horizon around midsummer, so astronomical darkness simply does not happen for weeks at a time. This page says so rather than printing a time that does not exist.
The Moon is usually the problem
A bright Moon floods the sky with scattered light and hides everything faint. If you want to see the Milky Way, a meteor shower or anything through binoculars or a small telescope that is not itself a planet, the week either side of new moon is worth waiting for — so moonrise and moonset matter as much to a night of stargazing as sunset does.
Moon phase is only the angle between the Sun and the Moon as seen from here — nothing more. The Earth's shadow plays no part in it, which is the single most common misunderstanding about the Moon. The shadow is a lunar eclipse, and that happens only a few times a year, because the Moon's orbit is tilted about five degrees and it usually passes above or below the shadow rather than through it.
How the positions are worked out
The Sun comes from a short trigonometric series accurate to about a hundredth of a degree, which is roughly a fortieth of the Sun's own width. The Moon is the hard one: the Sun pulls it around hard enough that a simple ellipse is useless and a full theory runs to thousands of terms, so the two dozen largest are used here, giving about a tenth of a degree. That is plenty to point at it and nowhere near enough to time an eclipse.
The planets use the Keplerian elements JPL publishes for approximate positions of the major planets, good to a few arcminutes between 1800 and 2050. Each planet is treated as though it orbits the Sun alone, with slow drift rates absorbing the pull of the others — a very good trick over two centuries and an increasingly poor one outside them.
Rising and setting without a formula
There are closed-form equations for rise and set times, and they all need special cases: one for the Moon, which moves too fast during the day it is being computed for; one for high latitudes, where objects need not rise or set at all; and different horizon corrections for each body. Instead this steps through the night, watches for the altitude to cross the horizon, and narrows each crossing down by halving the interval.
That is cruder and considerably sturdier. The same few lines work for the Sun, the Moon, a planet or a star; the polar cases fall out correctly rather than needing to be handled; and it cannot quietly return a plausible wrong answer, because the answer is checked against the actual altitude at that moment.
What the horizon corrections mean
Sunrise is defined as the moment the upper edge of the Sun appears, not its centre, and the atmosphere bends light over the horizon by roughly half a degree — so the Sun is geometrically below the horizon when you first see it. Together that puts sunrise at an apparent altitude of −0.833°.
The Moon needs a different correction again, because it is close enough that your position on the Earth's surface rather than at its centre shifts it by about a degree. Stars and planets get refraction alone. These are small numbers that shift the times by minutes, and getting them wrong is what makes an otherwise correct calculation disagree with the almanac.
Frequently asked questions
Do I need to give it my location?
You can, and it never leaves your browser if you do. Otherwise search for the nearest city — anywhere within a hundred kilometres or so gives times within a few minutes and directions that are indistinguishable by eye.
What time zone are the times in?
Your own, as your browser reports it, and the zone is named on the page. If you look up a city in a different zone the times are still shown in yours, which is usually what you want when you are working out when to call someone.
Why can I not see a planet it says is up?
Check the altitude. Anything under about 10° is behind whatever buildings, hills or trees you have, and is being seen through a great deal of atmosphere besides. Mercury is nearly always in this category — it never gets more than about 28° from the Sun, so it is only ever visible low in twilight.
Does it show satellites or the space station?
No. Satellite passes need orbital data that goes stale within days, which would mean fetching it from a server and would break the promise that this works offline. Planets and the Moon are predictable for centuries from numbers that fit in the page.
How accurate is it?
About a hundredth of a degree for the Sun, a tenth for the Moon and a few arcminutes for the planets. Rise and set times land within a minute or so. That is far finer than anyone can point, and not fine enough for occultation timing.
Does it work without a connection?
Yes. Once the page has loaded, everything is computed from orbits held in the page itself. Nothing is requested, which is deliberate — the moment you most want this is usually the moment you have no signal.