It is a star, is it a planet or is it a plane? A handy guide to identifying that bright object you saw last night.
The chances are it’s Jupiter or Venus (or just occasionally Mars). Planets will appear to move across the sky as the Earth turns, keeping their position with respect to the surrounding stars. Unlike stars, they generally don’t twinkle as they have an apparent diameter large enough that the effect of turbulence in the Earth’s atmosphere averages out.
Jupiter and Venus can both be strikingly bright. At maximum brightness Jupiter is four times and Venus 19 times as brilliant as Sirius, the next brightest star after the Sun. Once every 17 years Mars can be as bright as Jupiter (the last time was in 2003).
If the object you saw twinkled (possibly appearing to change colour as it does so) then it was probably a star. For example, in winter in Britain Sirius is visible fairly low in the south and shows exactly this behaviour.
Aeroplanes and satellites
Closer to home, if an aeroplane is flying directly towards you, it can appear to be stationary for a while (although the flashing landing lights may be visible) and can also be confused with a star or planet. However at some point the aircraft will appear to veer sideways or upwards as it passes by.
Many artificial satellites are also visible to the unaided eye and can be brighter than many stars. Satellites typically take about two minutes to cross the sky from one horizon to another. They are silent and do not have flashing lights. They fade out if they enter the Earth’s shadow.
Comets, meteors and meteorites
Although visible comets with their spectacular tails, are quite rare and you would be wise to look up the next expected visit, they do leave a large quantity of space debris in their wake. When the Earth passes through these debris fields, we see meteors or shooting stars as these bits of dust burn up in our atmosphere.
Occasionally a larger object smashes through the atmosphere and hits the Earth. This is a meteorite and will have come from an asteroid.
The comet 46P/Wirtanen is visible in our skies this month. Our astronomers will be looking to spot the Comet through the Annie Maunder Astrographic Telescope at the Royal Observatory.
Disappointingly, perhaps, UFOs don’t exist. Or do they? The truth is out there.