10 most difficult Moon riddles

The new moon is the first phase of the Moon, when its orbit is not seen from the Earth; it is at this moment when the Moon and the Sun have the same ecliptical longitude. The Moon is not visible at this time except when it is seen in silhouette during a solar eclipse when it is illuminated by earthshine.

Each of the subsequent five Apollo program landings were allotted gradually longer periods for EVA activities – the crew of Apollo 17, by comparison, spent over 22 hours exploring the lunar surface.

Armalcolite is a titanium-rich mineral with the chemical formula (Mg,Fe2+)Ti2O5. It was first found at Tranquility Base on the Moon in 1969 and is named for Armstrong, Aldrin and Collins, the three Apollo 11 astronauts. Together with tranquillityite and pyroxferroite, it is one of three new minerals that were discovered on the Moon.

On 17 September 2018, it was announced that Maezawa will be the first commercial passenger to attempt a flyby around the Moon. He will fly onboard a SpaceX Starship (formerly called BFR), which is currently in development. The flight is slated to take place no earlier than 2023 with a duration of nearly six days. He will take six to eight artists with him as a part of an art project he has created called #dearMoon.

If the rotational frequency of a satellite is larger than orbital frequency, a small torque counteracting the rotation arises, eventually locking the frequencies in a tidal lock. Pluto and Charon are the extreme example, being locked to each other.

In 1959, the Soviet Luna spacecraft photographed the invisible side of the Moon, proving that the dark patches on the surface are not seas of liquid, but only areas of fine volcanic dust.

The Moon's orbit is elliptical, and its apparent diameter varies by 10%. This is why both total eclipses (if the Moon is larger) and annular eclipses (if the Sun is larger) happen.

A blue moon is an additional full moon that appears in a subdivision of a year: either the third of four full moons in a season, or a second full moon in a month. Owing to the rarity of a blue moon, happens every two to three years, the term is used colloquially to mean a rare event, as in the phrase "once in a blue moon".

Mons Huygens is the Moon's tallest mountain (but not its highest point). It is about 5,500 m (18,000 ft) high and is located in the Montes Apenninus. The mountain was named after the Dutch astronomer, mathematician and physician Christiaan Huygens, it was officially named by the IAU in 1961.

Moonquakes are produced by tidal forces. Moonquakres are much less common and weaker than earthquakes, although they can last for up to an hour - a significantly longer time than terrestrial earthquakes - because of the absence of water to damp out the seismic vibrations.