Why is a blue moon called so?


          Will a month have two full moons? Sounds almost impossible. Let’s take a look. The Moon takes almost twenty nine and a half days to change from one full moon to another. But, an average month has a span of thirty days. This makes it very unlikely for a month to have two full moons.



          However, some months do have two full moons and it happens very rarely. If there are two full moons in a month, the second full moon is called ‘blue moon.’ This name is not linked to the colour. In a century, there will be forty-one blue moons.



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How does a lunar eclipse occur?


          For a while, we have been talking about sunlight illuminating the Moon. What would happen if something blocks the sunlight that it receives? Will the Moon vanish from the night sky?



          In some instances, the Earth blocks the sunlight that is supposed to fall on the surface of the Moon. This happens when the Earth’s shadow falls on the Moon as it comes between the Sun and the Moon. This phenomenon is known as lunar eclipse.



          Lunar eclipse happens only during the full moon when the Earth either blocks sunlight completely or partially. Depending on this, lunar eclipse can be total or partial. However, the Moon will not disappear completely during the eclipse. This is because of a small amount of indirect sunlight that falls on it.



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Why does the Moon take more time to change from a new moon to the next new moon than the time taken for one trip round the Earth?


          It takes almost 27 days for the Moon to complete one trip around the Earth. This is known as the sidereal month. But it takes almost 29 days to change from a new moon to the next new moon. This is known as the synodic month. There is a difference of roughly two days. Wonder why?



          We already know that the Moon’s phases change according to the sunlight that falls on it and new moon occurs when the Moon is placed between the Earth and the Sun.



          While the Moon moves around it, the Earth moves around the Sun. The Earth moves farther in space during the time the Moon completes one orbit. So, for the Moon to come between the Earth and the Sun again, it has to move a little farther. It takes two days for the Moon to travel this extra distance.



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How is waxing different from waning?


          The shape of the Moon changes throughout the lunar month. The period in which the new moon grows to a full moon and back to another new moon is known as a lunar month.



          A lunar month is characterized by waxing and waning. Waxing occurs between the new moon and full moon, when the moon grows in size. The side of the Moon illuminated by the sunlight becomes bigger; it grows from a waxing crescent to waxing gibbous and then to full moon. During the gibbous phase, 51-99 per cent of the moon’s surface is illuminated.



          On the other hand, waning occurs between the full moon and new moon. When the Moon is waning, its surface illuminated by the sunlight decreases day-by-day. The Moon changes from full moon to the waning gibbous phase. It then changes to the waning crescent and finally the new moon.



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How is a full moon different from a new moon?


          When the Moon and Sun are placed at opposite directions from the Earth, the sunlight falls on the side facing the Earth. Therefore, the Moon appears as a round shining disc and is known as full moon.



          On the other hand, when the Moon comes between the Earth and the Sun, sunlight falls on the side that faces away from the Earth. During this time, the side facing the Earth appears dark and is known as new moon. The sky looks bright during the full moon while the Moon will not be visible during the new moon, making it dull.



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Why does the Moon have different phases?


 



          The Moon does not appear in the same shape every day, it is ever changing. On some days, it looks like a shining disk, on some days it looks like an elongated C. These different shapes of the Moon are called the phases of the Moon.



          The orbiting of the Moon around the Earth is the secret behind the different phases of the Moon. We already know that the Moon has no light of its own. As the Moon moves around the Earth, different parts of it are lit by the Sun, thus causing different phases. In a fortnight’s time, the Moon goes from full moon to new moon.



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What is the interior of the Moon composed of?


          The Apollo astronauts placed instruments known as seismographs on the Moon to study about its interior. What we know about it now comes primarily from their studies.



          The Moon is made up of different layers like any other celestial body. It has an outer crust, an inner mantle and an innermost core. The outer crust is hard, dry and dusty. It is almost 60 kilometres thick in the near side while it is almost twice as thick in the far side.



          The mantle has a thick layer of rock at the exterior and is semi-rigid in the interior. The mantle runs down almost 800 kilometres. The Moon’s core is constituted of rocks rich in iron.



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What is lunar soil known as among scientists?


 



          Scientists have special terms to indicate each scientifically relevant object around us. At times, these words may confuse an ordinary person as scientists often use big words that are difficult for a common man to understand. For instance, the scientific term for dry skin is xerosis. Similarly, lunar soil is known as lunar regolith among scientists.



          Lunar regolith is formed as a result of comets or meteorites that fall on the surface of the Moon. It is rich in oxygen and various metals. The thickness of the regolith varies from one place to another; it is three to five metres thick in the Maria, while the thickness in the highlands is ten to twenty metres.



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Why does the Moon have extreme temperatures?


          The temperature on the Moon is not constant; it varies from boiling hot to freezing cold, depending on the position of the Sun. The Moon has no atmosphere to either trap the heat or to insulate its surface.



          A day on Earth is 24 hours. Daytime on the Moon is thirteen and a half Earth days long. A long night of thirteen and a half days follows a Moon’s day. When sunlight hits the surface of the Moon during the day, the temperature can go up to 127 degree celsius. In contrast, the temperature on the Moon can drop to minus 173 degree celsius during the night.



          The temperature at the Moon’s core is much higher than its surface temperature. However, there are regions on the Moon that never receive sunlight. This is because of the slight tilt of its axis. Unlike Earth, the Moon doesn’t have different seasons either.



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Would the compass point towards the north on the Moon?


          If you take a compass to the Moon, it would not point northwards. It is not because your compass is faulty. A compass shows directions in alignment with the Earth’s magnetic field. We already know that the Moon has a very weak gravitational field. It does not have a strong magnetic field either. To understand this better, let us take a look at how the Earth’s magnetic field is formed.



          The core of the Earth is filled with hot liquid iron. This molten iron swirls as our planet rotates, thus producing the Earth’s magnetic field. The Moon’s core is less dense than the Earth. Therefore, the Moon’s core may contain only a little amount of metal; this might not even be heavy iron. On the Moon, the fluid motion of the conducting material is either very less or not present at all. As a result, the Moon has a weak magnetic field compared to that of Earth. It is around one hundredth of the Earth’s magnetic field.



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What is libration?


 



          The Moon has a dark side. Don’t worry; we are not talking about anything evil here. The side that faces away from the Earth is known as the Moon’s dark side. This is because of the shape of the lunar orbit.



          We know that the lunar orbit is elliptical. As a result, the Moon travels at different speeds at different points. Because of this, we can see more than half of the Moon’s surface. This phenomenon is known as libration. About 59 per cent of the Moon’s surface is visible from the Earth because of this phenomenon. Johannes Hevelius studied librations in detail and published his findings in 1647. His work was titled Selenographia.'



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Why are the Earth and the Moon referred to as the double planet?


          Many planets in the solar system have natural satellites. But Earth and its natural satellite share a special relationship.



          The Moon is Earth’s only natural satellite. Though our Moon is small when compared to the moons of Jupiter or Saturn, its size in relation to that of Earth is quite large.



          Along with revolving around the Earth, the Moon also revolves around the Sun in sync with the Earth. They look more like two sister planets than a planet and its satellite.



          Therefore, together, the Earth and the Moon are considered as a double planetary system.



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How is the near side of the Moon different from its far side?


          The Moon has a near side and a far side. The side that permanently faces the Earth is known as the near side while the side that faces away is known as the far side. Though these sides are so, the amount of sunlight they receive is almost the same.



          The surface of the near side differs from the far side; the near side is covered by large patches of dark areas. Early scientists proposed that these dark areas were seas with water. The dark patches are still known as seas. The far side does not have such seas. The crust here is thicker than the side facing the Earth.



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How long does the Moon take to complete one rotation?


 



 



          The Earth takes 24 hours to complete one rotation. But the Moon’s rotation is not that short; it takes 27 days, 7 hours and 43 minutes for the Moon to complete one rotation.



          The Moon’s axis is similar to that of the Earth; it is tilted towards one side. The Moon completes one round in its orbit around the Earth in almost the same time taken for one spin.



          Since the time taken for both movements are the same, the Moon always keeps the same side facing the Earth. This means that we get to see only one side of the Moon.



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What are perigee and apogee?


          The Moon’s orbit is elliptical; it is shaped like a flattened circle. Because of this shape, the distance between the Moon and the Earth is different at different positions and it varies as it moves around the Earth.



          When the Moon is closest to the Earth on its orbit, it is said to be at perigee. The word perigee is of Greek origin and means ‘near Earth’. At perigee, the Moon is 356,400 kilometres away from the Earth.



          The word apogee comes from a Greek word that means ‘far from Earth’. When the Moon is at the farthest point from Earth in its orbit, it is said to be at apogee. At apogee, the Moon is 406,700 kilometres away from the Earth.



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