How Vesper works

Every night, millions of people notice a bright light in the sky and wonder what it is. The answer is knowable to the minute, for any spot on Earth. This site exists to hand you that answer without making you learn astronomy first.

Calculated, not looked up

Nothing here comes from a feed or a forecast. The positions of the Sun, Moon and planets are computed from the same orbital models astronomers use, accurate to about one sixtieth of a degree, which is far finer than your eye can tell. Star positions come from the Yale Bright Star Catalogue.

For your city, we step through the night in five-minute slices and ask, for each planet: is it above the horizon, is it high enough to clear rooftops, and is the sky dark enough for something of that brightness to show? Venus can be seen in bright twilight; Saturn needs proper darkness. The times you see are the answers to those three questions.

What "visible" means here

We assume a clear sky and a reasonably open view. We can't know about your clouds, your neighbour's tree or the hill to your west, so treat “low on the horizon” as a prompt to find an open spot. City lights don't hide planets or the brightest stars, so everything listed as easy really is easy, even from a city centre.

Your location

To show a sky on the home page we make a rough guess at your nearest city from your internet connection. If you press “Use my location”, your coordinates are used in your own browser to pick a page and are never stored on a server. The last city you viewed is remembered on your device so the other tools can use it. There are no accounts, no adverts and no tracking.

Credits

Planetary positions use the open-source Astronomy Engine by Don Cross. Star and constellation data are adapted from d3-celestial by Olaf Frohn. City coordinates are from Natural Earth.