For centuries, humans have written books, songs, and poems about the so-called “dark side of the Moon”, but only today did a man-made object make physical contact with the rugged, unexplored area. The news, revealed in the early hours of Thursday, that China’s space agency successfully landed the unmanned Chang’e-4 probe on the far side of the Moon prompted the appropriate levels of excitement, with photographs of its arid landscape beamed over to Earth within minutes. Announcing the momentous step, Chinese state media said the landing “lifted the mysterious veil” from the far side of the Moon and “opened a new chapter in human lunar exploration”.

But for those of us without any expertise in space travel, the news raised as many questions as it answered. Haven’t we already landed on the Moon? What will this new mission teach us? And what exactly is the ‘far side of the Moon’, anyway?

Andrew Coates, professor of physics at UCL’s Space Science Laboratory, answers some common questions.

What is the ‘far side of the Moon’?

The ‘far side of the Moon’ (or “dark side of the Moon” as it is more commonly known, thanks in no small part to the 1973 Pink Floyd album) refers to the previously unexplored area of the Moon’s surface invisible from Earth. It’s all explained by ‘tidal locking’: the Moon moves in gravitational lockstep with the Earth, rotating at the same rate that it orbits our planet, which means only one section of the Moon is ever visible from Earth (the ‘Earth-facing side’). Over time, we can see slightly more than half of the Moon’s surface from Earth, says Prof Coates, due to small variations in the Moon’s orbit speed (a process known as ‘libration’).