Frequently-asked questions about the Moon


* Las preguntas más frecuentes sobre la Luna

The Moon, despite being an everyday object, still holds many mysteries. Not to mention many myths and misinterpretations. Here are the most popular questions about it.

"Why are there days when there is no moon?"

The Moon revolves around the Earth, sometimes it is to one side of our planet and sometimes at the other side. When it is not seen from Japan it is seen from Paraguay.

"Can we see the moon during broad daylight?"

Yes, we can. In fact the Moon is visible during daylight the same amount of time that is visible during the night. During daylight hours, stars other than the Sun are not seen because the air takes on a strong blue glow caused by sunlight, but the Moon is brighter than this air glow.

"When will it be full moon?"

If we look at the Moon and the Earth together in space we see that both always have 50 % of its surface area darkened, and 50 % illuminated by the Sun. And this happens with all the planets. As the Moon goes around the Earth, sometimes we are in front of our natural satellite, sometimes we are behind it, or to one side or the other, seeing its illuminated portion or its dark portion, or a little of the dark portion or a bit of its lit portion. Those are what we call phases. The Moon takes about a month to revolve around the Earth.

The "new" Moon rises in the east at sunrise, and sets to the west at sunset (it goes in the sky right along with the Sun, so it is difficult to see). A week later, "waxing" Moon, it rises at noon, it trails the Sun (at a distance of 90 degrees) and sets at midnight. In the third week, "full" Moon, it rises in the evening (at 180 degrees from the Sun) and sets at sunrise. In the fourth week, "waning" Moon, it rises at midnight and goes ahead of the Sun (90 degrees ahead, the final ones) and sets at noon. And so on to "new" Moon again, having gone around the full lap.

By very accurately measuring its position relative to the horizon, the Sun and other stars, ancients peoples were able to understand this and other movements of the Moon.

"What is a 'Blue Moon'?"

It refers to two full moons in the same month. The period between two full moons is 29,5 days, so this can happen. The term comes from agricultural almanacs, like terms such as the Hunger Moon (any full moon in snowy February), Flower Moon (in springtime May), Harvest Moon (in September), etc.. The first calendars were lunar calendars, and many are still in use. For example the liturgical calendar (derived from the Jewish calendar) determines our Easter holidays and even the Carnival season.

There are other terms that nonetheless are clearly derived from astrological quackery, for example the so-called (wrongly-called) "supermoon". It is not a specific cycle nor a specific periodicity, nor is a component of the celestial mechanics of the body, but simply an ephemeris position. It refers to the observation of the "full" Moon at a time when it is actually closer to the Earth than the rest of the time, so it looks a little bigger and brighter than other full moons. Of course, this is no cause for disasters or anything even remotely like that, although for astrologers it is time to line their pockets. In scientific terms the right (and only) thing to state is that the Moon is in opposition near its periapsis.

"What's that bright point of light that is always next to the Moon?"

It is usually the planet Venus. We tend to look at the Moon only when it is near the horizon and during twilight. As Venus orbits perennially at a relatively short distance from the Sun, we see it under those same conditions. This planet, the closest to our planet Earth, is four times larger than the Moon, but our natural satellite is much closer to us. On these occasions we also see, on the dark portion of the Moon, a dim illumination: it comes from the Earth. In fact our planet as seen from a distance is at least four times brighter (usually much more brighter) than the Moon.

"That dark portion of the Moon, is the Earth's shadow?"

No, is the shadow of the Moon itself. It's the part where there is no sunlight, where is night on the Moon. The Earth's shadow falls on the Moon very rarely, once a year or so. The rest of the time the Moon passes slightly above or below the Earth's shadow. This is why there are no lunar eclipses every month. If the Earth were like an apple, the Moon would be like a grape but separated by two meters. It's very difficult to obtain an exact alignment of the shadows. When it happens, in full or new moon, the Earth's shadow falls on the Moon or the Moon's on Earth. The twilight-red shadow of the Earth is four times larger than the Moon. We can see the lunar eclipse from any place in the world provided the Moon is up and over the local trees and hills at the time of occurrence. But the shadow of the Moon is smaller than planet Earth, so that only some rather narrow regions of the world go into darkness during each solar eclipse.

"Why does the Moon appear larger when near the horizon?"

It is a brain illusion caused by seeing the Moon among a foreground landscape. Our naked-ape brains are not fit enough to look at such a dismally distant object and make sense of it among pedestrial things, so they instinctively try to "get the Moon closer". If we look at the Moon through a tube, hiding the landscape, it seems of normal size. The actual size was measured for the first time by the ancient Greeks: they first measured the Earth, by noting the sunlight falling at different angles on cities separated by known distances; then, by watching the shadow of the Earth during an eclipse they were able to calculate the diameter of the Moon: almost 3500 km. And even more: by taking note of the angular size of the Moon they obtained the distance to that celestial body, about 380 000 km. Applause to the naked apes!

"What are those dark spots on the Moon?"

They are great frozen lava plains. Galileo called them seas, a denomination that continuous on today's selenography maps. With a telescope we can observe that there are also mountains as high as those on Earth, including spectacular lunar mountain ranges; they were given names after mountain ranges from here on Earth. And there are also craters, some of them hundreds of kilometers in diameter; these usually have names after great scientists and philosophers. There are even lava marks resembling somewhat river beds.

"Why does the Moon have so many holes?"

Most are marks of impacts by asteroids and comets that fell on the Moon over thousands of millions of years. They also fell on Earth, but here there is rain, wind and tectonic movements so those marks disappear. On the Moon there is none of that, so the craters remain.

"Is the Moon hot or cold?"

It is both. As there is no air, there is nothing to filter the Sun's heat and there is nothing to retain that heat in the shade. Temperature varies from -150 C to +120 C.

"Does the moon influence hair growth?"

No, it does not. Its only influence are its gravity and its reflected sunlight. In the case of the entire Earth one side is much closer to the Moon than the other, therefore there is a difference in the pull of gravity and our planet becomes deformed. It stretches 30 cm on the continents and several meters on the oceans. But the distance between the tip of the hair and the head scalp is minimal, so a strand of hair is not stretched appreciably. Light influences predators' habits, but it does not foretell a person's future.

"Can we see the American flag with a telescope?"

No, we can not. The Moon is so far away that even with the best telescopes we can not see objects less than 100-m wide. In order to see more details we need spacecraft that travel and reach up there, close enough. And also to see the other side: due to Earth's gravity, one side of the Moon was "locked" toward the Earth so that we always see the same set of spots.

"Is it a hoax that Americans landed on the Moon?"

No, it's true. The immense rocket-science project lasted for 12 years and 20 thousand million dollars were spent on it. Almost 400 000 people worked on it manufacturing millions of pieces, and many of the remaining rockets and spaceships can still be seen in museums in the U.S. and abroad. In addition, with the six successful landing missions, almost half a ton of Moon rocks (impossible to counterfeit) were brought to Earth and distributed to dozens of labs around the world.

"What is the moon made of?"

Of about the same things as the Earth: oxides of silicon, magnesium, calcium, aluminum. There is less iron and more titanium than here. It has crust and mantle but almost no core. All these suggests that the Moon would actually be a piece of the Earth: at the time of formation, this would had been hit by a planet the size of Mars, which tore the top of it. The remains were thrown into space and the Moon formed when these re-aggregated in orbit. It's there by accident. In fact, it is abnormally large.

"Is there water on the Moon?"

There is no air pressure to keep the molecules together and the daytime heat is excessive, but in the dark bottom of craters near the poles there are cometary H2O ice crystals mixed with soil, about 3 kg per ton. It might provide water for drinking, and from it even oxygen for breathing and hydrogen for fuel, to the future human colonies.

Aldo Loup.

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Based on a talk originally given at USP, on 17 May 2003. Originally published in ABC Color, on 30 April 2006. Photograph: This large stone in the sky we named "Luna". We are very lucky to have it: it apparently originated in an accident, thousands of millions of years ago. In this nearly full-phase-view of our neighboring world (centered in the crater Kästner [7 deg S, 78 deg E], with the rims of craters Ptolemy and Aristarchus barely visible on the upper limb [apparent contour] of the Moon), the dark circular plain (in the right half of this hemisphere) is the Sea of ​​Crises; above it is the Sea of ​​Tranquility, and at the right end of this, part of the Sea of ​​Serenity appears; to the left of the Sea of Crises is the Sea of Fertility (with the bright crater Langrenus on its "shores") and above it, smaller in size, the Sea of ​​Nectar; in the left region of the photo is the lunar South Pole and in the lower half of the photograph we see two minor basaltic plains, the Sea of ​​Smyth and to its right the Marginal Sea, marking the limits of how far out the lunar landscape we can normally see from Earth: the lower third that lies beyond, toward the longitudes of the dark crater Tsiolkovskiy with its kilometers-tall central mountain, among the long shadows of the terminator (where the lunar day ends and the lunar night begins), belongs to the far side of the Moon and is only visible from spacecraft. Credit: taken around 1 A.M. on 17 December 1972 (UTC) by the travelers of Apollo 17 (Gene Cernan, Ron Evans and Harrison "Jack" Schmidt, probably by the second one), about 5000 km off after having departed from a close-to-perigee Moon and still distant about 350 000 km from their homes on Earth, with a Hasselblad 500 EL camera, HEC space modified version, configuration -210 (without the glass Reseau plate, that leaves the famous fiducial crosses for photogrammetry), with an 80,250-mm, f/2,8 Carl Zeiss Planar lens, -223 version, using 70-mm-wide Eastman Kodak SO-368 Color Exterior (CEX) Ektachrome MS film, sensitivity to light ASA 64, and subsequently cataloged in files as coming from the roll cartridge number 152 (also identified as "PP"), photographic frame 23311 ... photographic frame that thus has become a faithful memory of such an incredible human feat. Courtesy Johnson Space Center / NASA.