What is that bright star? How to tell a planet from a star
The brightest "star" in the sky is usually a planet. Six quick checks tell you whether you are looking at Venus, Jupiter, Mars, Sirius, a satellite or a plane.
Updated
If one light in the sky is far brighter than everything around it, the odds are good that it is not a star at all. Venus and Jupiter both outshine every star in the night sky, and Mars does too when it is close to Earth. Here is how to work out what you are looking at in under a minute.
1. Is it moving?
Watch it against a rooftop or a branch for ten seconds.
- Crossing the sky in a few minutes, steady light, no blinking: a satellite. If it is very bright, it is probably the International Space Station.
- Moving, with blinking red, green or white lights: an aeroplane.
- Not moving that you can tell: a star or a planet. Keep going.
2. Does it twinkle?
Stars twinkle. Planets mostly shine with a steady light.
A star is so far away that it is a single point, and the moving air above you bends that point of light this way and that. A planet is close enough to be a tiny disc, which smooths the shimmer out. The test is less reliable near the horizon, where everything shimmers through thick air.
3. When and where are you seeing it?
- Blazing white, low in the west after sunset or low in the east before sunrise: Venus. It is the brightest thing in the sky after the Sun and the Moon. Because it orbits closer to the Sun than we do, it never strays far from the Sun, so you never see it high in the sky in the middle of the night.
- Very bright, cream-white, up for much of the night: Jupiter. It can be high overhead at midnight, which Venus never is.
- Orange-red and steady: Mars. Its brightness changes a great deal over about two years, from unremarkable to brighter than any star.
- Pale yellow, steady, as bright as the brighter stars: Saturn.
- A small point very low in bright twilight: possibly Mercury, which is the hardest of the five to catch.
4. Is it on the planets' line?
The planets, the Moon and the Sun all travel along the same band of sky, called the ecliptic. If your bright light sits roughly on the arc the Sun followed during the day, or the one the Moon follows at night, a planet is likely. A bright light far from that arc, for example high in the north as seen from Europe or North America, is a star.
5. Could it be Sirius?
Sirius is the brightest true star, and it fools people every winter. When it is low it flashes red, blue and green as it twinkles, so strongly that people often take it for something else. From the northern hemisphere, look for it below and to the left of Orion's Belt: the three belt stars point down toward it.
Other bright stars that draw attention: orange Arcturus in spring, blue-white Vega overhead in summer, yellow Capella high in winter, and from the southern hemisphere Canopus and Alpha Centauri.
6. Check tonight's sky for your city
Which planets are up changes from month to month, so a fixed list would soon be wrong. The tonight's sky pages work it out for your city every day: which planets are visible, when each is best placed, where to look, and which bright stars are up with them.
The quick way
Point your phone at it. Star & Planet Finder names whatever is in the centre of the screen, and its arrow leads you to any planet or star you search for.
