The fascinating issue of look-back times. Because of the finite speed of light, when you gaze up into the night sky, you are looking into the past. The bright star Sirius is 8.6 light years away. That means the light hitting your eye tonight has been traveling for 8.6 years. Put another way: When you look at Sirius tonight, you see it as it was 8.6 years ago. As you look at more distant objects, the effect becomes bigger and bigger. The stars of the Big Dipper range from 60 to 125 light years away. When you look at Dubhe, the front star in the “bowl” of the Dipper, you are seeing light from before you were born.

When we look up at the stars, we are looking back in time. The light entering our eyes from these distant objects set off years, decades or millennia earlier. Every time we look at something ‘up there’ we’re seeing it as it was in the past.

As far as our own Solar System is concerned, one of the farthest objects observed so far is a 500km-wide asteroid, 2018VG18. Also known as Farout, it is over 120 times further from the Sun than Earth, or three times further away than dwarf planet Pluto. If you were standing on Farout’s dark, pink icy surface, Earth would be a mag. +6.8 ‘star’ hugging close to the Sun and Jupiter would shine at barely mag. +5.0. If a space probe ever reaches Farout it will take 17 hours for its radio signals and images to reach Earth.

If you don’t count the International Space Station or the shining swarms of satellites orbiting Earth, the Moon is our closest celestial neighbour. A beautiful sight in the night sky, it is often seen shining close to bright planets in an event known as a conjunction. It's also a beautiful object to image. For more on this, read our guide on how to photograph the Moon or our beginner's guide to astrophotography. Although only 384,400km away it still took Apollo crews around three days to reach the Moon. But light, travelling at 300,000 km/s, can cross that distance in just under 1.3 seconds.

So when you look at the Moon you’re seeing it as it was a blink of an eye ago…

Heading outward into the Solar System, past Mars and across the asteroid belt, we eventually come to the most massive planet in the Solar System, Jupiter. The gas giant can be between about 600 million km and 970 million km from Earth, depending on what point its is currently at in its orbit. When you look at it, you’ll be seeing it as it was about 48 minutes earlier. That’s not inconvenient for us stargazers, but engineers controlling the Juno space probe, currently orbiting Jupiter and sending back stunning photographs of its swirling cloud bands, do have to take the time delay into account. It means having to plan imaging sequences very carefully and with great accuracy, well in advance.

Visible to the eye on a dark, Moon-free night as an extended smudge between Cassiopeia and the Great Square of Pegasus, M31, the Andromeda Galaxy, is a vast spiral galaxy, perhaps twice the size of our own Milky Way, and more than 2.5 million lightyears from it. When we look at M31 we are not just staring across a gulf of space, but far back in time too, to when our Homo habilis ancestors were using sophisticated stone tools in East Africa, and the UK was buried under sheets of ice.