How did the moon get this apparent 'X' mark on its surface?

The Moon appears to have a fairly large "X" mark on its ring-covered surface, but while it may look intriguing when viewed from Earth, it doesn't exactly "mark the spot" of any particularly interesting features on the lunar surface.

In fact, it's an illusion created when light hits the rims of three neighbouring craters, named Blanchinus, La Caille and Purbachm at the correct angle along the shadow line between lunar day and night.

The phenomenon is visible only for a few hours every lunar month just before the moon reaches its first-quarter phase. The 'Lunar X' can be seen through binoculars or small telescopes.

How we went to the moon
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How we went to the moon

Aldrin walking on the moon.

Aldrin stands near a scientific experiment on the lunar surface.

Aldrin salutes the flag they planted on the surface of the moon.

Apollo 11 astronauts (left to right) Aldrin, Collins, and Armstrong leaving crew quarters to enter the Astrovan for the ride to Launch Pad 39A.

NASA officials join the flight controllers in celebrating the conclusion of the Apollo 11 mission.

Apollo 11 lunar module rising above the moon to rendezvous with command module before heading home.

The American flag at Tranquility Base on the Moon, planted by the Apollo 11 astronauts.

Armstrong's footprint on the moon.

Spectators as they watch (and film) the launch of NASA's Apollo 11 space mission from a concrete ledge at Cape Kennedy (later Cape Canaveral) on July 16, 1969.

A ring of condensation forms around the Saturn V rocket as it compresses the air around it during the launch of Apollo 11.

A pre-launch twilight photo of the The Apollo 11 Saturn V space vehicle.

Prelaunch breakfast in crew quarters with Armstrong, Collins, and Aldrin.

the International Arrival Building at John F. Kennedy International Airport shows a crowd of passengers as they watch a large screen television that broadcasts the Apollo 11 moon landing mission, New York

Vice President Spiro Agnew and former President Lyndon Johnson view the liftoff of Apollo 11 from the stands located at the Kennedy Space Center VIP viewing site.

Aldrin inside the Eagle on the way to the Moon.

Aldrin does science experiments on the moon.

The Command Module, as taken from the Lunar Module, during its orbit of the moon.

Apollo 11 mission officials relaxing in the Launch Control Center at Kennedy Space Center on July 16, 1969.

AS11-40-5902 (20 July 1969) --- Astronaut Edwin E. Aldrin Jr., lunar module pilot, walks on the surface of the moon near a leg of the Lunar Module during the Apollo 11 extravehicular activity (EVA). Astronaut Neil A. Armstrong, Apollo 11

Aldrin descends the steps of the Lunar Module ladder as he prepares to walk on the Moon.

A photo of Aldrin, taken by Armstrong.

Collins during Apollo 11's first around the Earth.

These Photos of the Apollo 11 Moon Landing Will Leave You in Awe

Armstrong after his historic walk on moon.

Fifty years ago, the Apollo 11 space mission took its place in global history.I remember watching the Apollo 11 moon landing. It was an amazing achievement, enabled by a brilliant team of engineers, scientists and technicians at Nasa. I was still at school and we were utterly awed by the engineering and ingenuity that made it happen. The posters on my wall at university were the famous shot of Neil Armstrong on the moon and the Blue Planet from space.Today, half a century later, it's important to remember how crucial the inspiration of that one small step was to a new generation of engineers around the world – it would underpin so many of the innovations we take for granted today. Here in the UK, a "new Britain" was being forged in the "white heat" of technology. 1969 saw the MacRobert Award for Engineering Innovation presented for the first time. Established by the MacRobert Trust, the medal features a man leaping for the moon to commemorate the lunar landing, and the £50,000 prize recognises those that meet three key criteria – commercial success, societal benefit, and true innovation.In a year that saw Americans on the moon, the judges had a tough decision as to who should win that first award for British innovation. They announced joint winners: a team from Freeman, Fox and Partners for the aerodynamic deck design of the Severn Bridge – later used for long-span bridges all over the world – and a team from Rolls-Royce for the Pegasus engine that powered the Harrier, the world’s first vertical take-off and landing aircraft.Since 1969, the global influence of winning British innovations has been maintained, with a host of world firsts, including the CT scanner in 1972, the first bionic hand in 2008 and Raspberry Pi, the world’s most affordable computer, in 2017.Landing on the moon gave the whole world a new perspective on our planet, in particular the fragility of our environment. It is only fitting therefore that our 50th anniversary winner is already reducing the carbon footprint of commercial flight – Bombardier’s advanced composite aircraft wing is the first certified commercial aircraft wing made using resin transfer infusion. This new technique, developed in their world-leading Belfast facility, uses less energy, fewer parts and results in a lighter wing. The carbon composite wing is approximately 10 per cent lighter than a metal wing, reducing fuel burn and emissions.As we celebrate the achievements of the Apollo programme’s engineers and astronauts in the late 60s, it is crucial that we continue to support the generations who carry forward their legacy.Inspired by innovation, today’s young engineers know that nothing is impossible in meeting the grand challenges we have to face in the future.Dr Dame Sue Ion DBE FREng FRS, is chair of the UK Nuclear Innovation Research Advisory Board and chair of the judging panel for the MacRobert Award for engineering innovation

According to Nasa's Robert Nemiroff and Jerry Bonnell: "An astronaut standing close to the craters' position would see the slowly rising Sun very near the horizon.

"Temporarily, crater walls would be in sunlight while crater floors would still be in darkness.

"Seen from planet Earth, contrasting sections of bright walls against the dark floors by chance look remarkably like an X"

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