Notes
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Topics to be learn :
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The moon’s motions :
- Like the earth, the moon also has axial and orbital motions. The moon while rotating around itself, revolves around the earth and the earth revolves around the sun.
- As a result, the moon indirectly makes revolutions around the sun.
- The time taken by the moon to make one revolution around the earth and one rotation around itself is the same. Therefore, we constantly see one and the same side of the moon.
Perigee and apogee :
- Due to the moon’s elliptical orbit of revolution, its distance from the earth is not the same everywhere along its orbit.
- When the moon in its orbit is nearest to the earth, it is said to be in perigee.
- When the moon in its orbit is farthest from the earth, it is said to be in apogee.
Click here to View Figure-1
Phases of the moon :
- The moon gets illuminated by the sunlight. The illuminated part of moon seen from the earth appears greater or smaller day by day due to the moon’s revolution around the earth.
- From the new moon to the full moon, the illuminated part of the moon goes on increasing gradually. Similarly, from the full moon to the new moon, the illuminated part of the moon goes on decreasing gradually.
- The changing shapes of the illuminated part of the moon as seen from the earth are called phases of the moon.
See fig, it shows the relative position of the moon on both the quarters, the full moon and the new moon days with respect to the earth and the sun.
Click here to View Figure-2
- Full moon day : On the full moon day, the sun’s light falls on the entire portion of the moon facing the earth. Therefore, the moon disc appears as a full circle.
- New moon day : On the new moon day, the sun’s light does not fall on the portion of the moon facing the earth. Therefore, the moon disc appears to be dark and invisible.
- Quarter days : On the first and the third quarter days, the sun’s light falls on the half portion of the moon facing the earth. Therefore, the moon disc appears as a semicircle.
Period of waxing moon : The fortnight from new moon to full moon in which the illuminated part of the moon seen from the earth keeps on increasing (waxing) is called period of waxing moon.
Period of waning moon : The fortnight from full moon to new moon in which the illuminated part of the moon seen from the earth keeps on decreasing (waning) is called period of waning moon.
The other side of the moon is never seen from the earth :
Reason :
Eclipses :
The orbital path of the earth and that of the moon : The orbital path of the earth around the sun and that of the moon around the earth are not in the same plane. The moon’s revolutionary orbit makes an angle of about 5° with that of the earth. As a result, the moon intersects the plane of the earth’s orbit twice during one revolution.
Click here to View Figure-3
New moon day and the lines joining the sun, the earth and the moon : The sequence of the sun, the earth and the moon on the new moon day is : Sun — Moon — Earth. Even so, the lines joining the sun, the moon and the earth may not make an exact angle of 0° on every new moon day.
Full moon day and the lines joining the sun, the earth and the moon : The sequence of the sun, the earth and the moon on the full moon day is : Sun — Earth — Moon. Even so, the lines joining the sun, the earth and the moon may not make an exact angle of 180° on every full moon day.
A reason behind non occurrence of eclipses on every new moon day and full moon day : On every new moon day and full moon day the sun, the earth and the moon may not be in one straight line in the same plane. Therefore, eclipses do not take place on every new moon day and full moon day.
Eclipses and the position of the sun, the earth and the moon : On some new moon day and full moon day the sun, the earth and the moon fall in one straight line in the same plane. Eclipses occur on such occasions. Eclipses may be solar or lunar.
Solar eclipses : If the moon is between the sun and the earth, the shadow of the moon falls on the earth. At such a time, these three celestial bodies are in the same plane and fall in one line. Hence the place on the earth where the moon’s shadow falls, experiences a solar eclipse. The shadow of the moon is of two types. The central portion of the shadow is darker and the periphery is light. Click here to View Figure-4
Lunar eclipses : If the earth is between the sun and the moon the shadow of the earth falls on the moon. At such a time, these three celestial bodies are in the same plane and fall in one line. A lunar eclipse occurs when the moon enters the shadow of the earth, while revolving around itself. Click here to View Figure-5
Characteristics of solar eclipses and lunar eclipses : (2) Occur if and only if the sun, the moon and the earth are in the same plane and fall in one line. (3) The maximum duration of a total solar eclipse: 7 minutes and 20 seconds (440 seconds). (2) Occur if and only if the sun, the earth and the moon are in the same plane and fall in one line. (3) The maximum duration of a total lunar eclipse : 107 minutes.
Solar eclipses
Lunar eclipses
(1) Occur on new moon day, but not on every new moon day.
(1) Occur on a full moon day, but not on every full moon day.
Eclipse — an astronomical event :
- A solar eclipse or a lunar eclipse is just an astronomical event. There is nothing auspicious or inauspicious about these events.
- Eclipses are just a result of the sun, the earth and the moon being in specific positions.
- There is lot of curiosity about eclipses, because an eclipse is not a regular occurrence.
Eclipse - an opportunity for study :
- Eclipses and particularly total or annular solar eclipses present great opportunities for study for scientists working in the field of astronomy.
- Scientists from all over the world make it a point to visit the locations where such events are going to occur. They carry out elaborate studies of eclipse conditions.
Eclipse — responses of birds and animals :
- During the period of solar eclipse, a large number of birds and animals get confused due to the untimely darkness that set in.
- Their responses to this event are unusual.
Solar Eclipse - Precautions :
- The intense light of the sun can be harmful to the naked eyes.
- It is necessary to view solar eclipses through dark glasses or special goggles made for that purpose.
| Occultation and Transit: Like eclipses, some other specific conditions occur with reference to the sun or the moon. These are called occultation and transit respectively.
Occultation occurs with reference to the moon whereas the transit is associated with the sun. Occultation: This is a typical event occurring in space. The moon revolves around the earth. While doing so, it obscures a star or a planet and that celestial body appears to hide behind the moon. This is called occultation. The total solar eclipse is actually a kind of occultation. During this, the sun disc gets hidden completely because of the moon. Transit: If an inner planet like Mercury or Venus comes in between the line of the earth and the sun, a transit occurs. At that time, a small dot appears to move across the sun disc. Transit is a type of solar eclipse. |