Why do we see the phases of the Moon? | The scientists respond


Photograph resulting from a mosaic of 40 individual images from different parts of the Moon.
Photograph resulting from a mosaic of 40 individual images from different parts of the Moon.

The Moon is a rocky object that does not have an atmosphere and does not emit light in the same way that a rock that we find in the field does not emit light. When we look at the sky at night, we only see the part of the Moon that is currently being illuminated by the Sun. It is something similar to walking in the field at night and lighting a stone with a flashlight. We only see the part of the rock illuminated by the light of our flashlight, the rest remains in the dark. In the case of the Moon, we are not the ones who are illuminating it with a supergiant flashlight but the Sun and that is why we cannot control which part we see. The relative position of the Sun, the Earth and the Moon determines that we see a greater or lesser fraction.

We all know that the Moon revolves around the Earth. When we are at that moment of the turn in which the Sun, the Earth and the Moon are approximately aligned in that order, from the Earth we can see the illuminated side of the Moon in full and it is what we call “full moon”. In the opposite case, when the alignment order is Sun, Moon and Earth we cannot see the face illuminated by the Sun and it is what we call “new moon”. The intermediate positions correspond to the waxing or decreasing moon.

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We speak of a crescent moon when every day we see a larger fraction of the illuminated face, in other words, it is the period that goes from the new moon to the full moon. The crescent moon is seen in the Northern Hemisphere as a D (arc lit to the right) and a C-shape (arc lit to the left) in the Southern Hemisphere. We say that the Moon is in a decreasing or waning phase when each day we see less portion, that is, it is the period that begins with the full moon and ends with the new moon. In the northern hemisphere the decreasing moon appears in the shape of a C, while in the southern hemisphere it appears in the shape of a D.

The lunar cycle lasts approximately 29 days and 12 hours. This is the time necessary for the same moon phase to be repeated, or in other words, for the same relative position of the Sun, the Earth and the Moon to be repeated. However, the time it takes for the Moon to complete an orbit around the Earth is somewhat less, approximately 27 days and 7 hours. The difference between the two times is due to the fact that while the Moon revolves around the Earth, the Earth is also revolving around the Sun. And the two cycles do not exactly coincide because the Earth “moves”.

In addition to the lunar phases, we terrestrials have other evidence that the Moon is rotating around our planet. The Moon, like the Sun and all the stars, rises and sets every day. Due to the rotation of the Moon around our planet, the time at which the Moon appears on the horizon is delayed by about 50 minutes each day, repeating the same time in approximately one lunar cycle.

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The cultural significance of the lunar phases is enormous. Since ancient times they have been used to create calendars. Already in Babylon they measured time with lunar months. Our months are also lunar, although we have rounded them by setting the duration at 30 days. And there are religious calendars, like the Muslim, that strictly follow the lunar cycles.

A question that we have all asked ourselves is why there is not an eclipse of the Sun every month. This is not the case because the plane in which the Moon rotates around the Earth is inclined about 5 degrees with respect to the plane in which the Earth rotates around the Sun. Every month we go through the new moon phase in which the Moon It is located between the Sun and the Earth, but only when the three stars are perfectly aligned does an eclipse of the Sun take place. Each solar eclipse is visible only from certain areas of the Earth. To see a total solar eclipse from the same place on Earth you have to wait hundreds of years. Fortunately, we have to wait less because in Spain a total solar eclipse can be observed on August 12, 2026.

The Moon is gradually moving away from us, at a rate of about 3.8 centimeters each year. There will come a time when it will look smaller and will no longer be able to outshine the Sun.

Asuncion Fountain She is a doctor in Theoretical Physics and researcher at the National Astronomical Observatory.

Question sent via email by Orfilia Beatriz Garrido

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Coordination and writing: Victoria Toro

We respond is a weekly scientific clinic, sponsored by the Dr. Foundation Antoni Esteve and the program L’Oréal-Unesco ‘For Women in Science’, which answers readers’ questions about science and technology. They are scientists and technologists, partners of AMIT (Association of Women Researchers and Technologists), those that answer those doubts. Send your questions to [email protected] or on Twitter #nosotrasrespondemos.

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George Holan

George Holan is chief editor at Plainsmen Post and has articles published in many notable publications in the last decade.

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