We have seen how the moon is closely associated with our religious life. In Islam it is mandatory that we measure the day by the sun and the months and the years by the moon. We have also said in passing the reliability of the moon’s motion as a determinant of larger measure of time. We will look into the physical aspects of the visibility of the moon and the causes for the apparent waxing and waning phenomena. Actually Moon sighting is a misnomer. It should be rendered as Moon observation.
Today we know, and the Qur’aan had declared it 1400 years ago, that the sun, Moon and the Earth are globular bodies and that they have their own fixed pattern or orbits such that no one ‘catches’ with the other or collides with another. Tradition has it that the Holy Prophet (saw) on his flight of Mi’rage saw the earth and the moon below as small spheres and this observation had a great deal to tell us. The simple inference is that the moon does not dissolve nor does it gain volume. Then the other inference that must follow is that its full or partial visibility is what gives us the impression of waxing or waning. What we are concerned with is the relative measurement of moon and the sun from the earth.
Though the Sun is a member of a larger group, constantly on the move, when viewed in the limited relation of the earth and the moon is static or stationary. The light that is emitted from the Sun illuminates both the earth and the moon. We all know that we can see an object if it is illuminated by beams of light which reflects on its surface and strike our pupils. Unlit areas on the object do not reflect light and are not visible. It can also be that the beams falling on the object reflect in a different direction and hence do not reach our eyes and are invisible to us. They could be visible to those who receive the reflected rays. As such we cannot deny the existence of an object simply because we do not see it. Such is the case with the moon.
When the earth comes in between the sun and the moon with the three in one line the earth prevents the light of the sun from falling on the moon. Moon cannot reflect any light and so it becomes dark. For a person on the earth looking at the moon there is one more limitation to take into account. That is the span of his vision. The span of his vision is limited on either side by the tangential line from the source of light to the extreme point on the two edges of the diameter of the earth. Rays passing beyond these points are not visible to him. See the illustration below.
Pictures of various phases of moon and Diagrams or movies to be inserted here
You can experiment this in a very simple manner. Switch off all lights in a room and close the doors and windows to prevent light coming from outside. Place a lighted candle on a table. Place an inflated ball, large enough to block the candlelight falling on the ball. Place or hold vertically a small coin or marble close to the ball and make the candle flame, the center of the ball and the center of the coin to fall in one line. Observe the amount of light falling on the coin. Slowly raise your hand until the bottom cone of the coin is in line with the top of the ball. As your hand rises you find that the top of the coin becomes brighter and gradually the whole coin looks brighter. This is exactly what happens to the moon when it is in the shadow and emerges from the shadow during a lunar eclipse. Imagine there is an ant on the ball and that it looks towards the coin. You can imagine how the coin becomes visible to it. Repeat the experiment by placing the candle with a pen torch and by suspending the ball from under a table or a chair. Ask someone to hold the lighted torch at various angles taking care that the light falls on the ball and creates a distinct shadow. Hold the coin vertically in the shadow and slowly move into the beams of light beyond the edge of the ball’s shadow for each position of the light.
Fix a few pins on one half (hemisphere) of the ball and hold the ball in such a way that all the pins are fully illuminated by the light from the candle or the torch. Take a long reed or a rib of a coconut leaf and hold it in such a way that it touches the pinhead vertex of the ball and top of the coin held vertically. This is the line of vision of the pinhead directed towards the coin. When the coin is in the dark the shadow of the reed will not touch the coin. But as it emerges from the darkness it will touch the coin. Hold the coin in such a way that the end of the reed touches the flat surface of the coin. You can see that the area above the reed is brighter than the rest. If you put your eye at the pinhead you can see the coin rising and gradually waxing. Repeat with different pinheads. You will find that for the pins away from the rim of the ball visibility is reduced and also that visibility is reduced or enhanced depending on the angle of the beam. Relating this experiment to the sun, Moon and the earth marking in their different planes we can easily understand that the visibility depends on the position of the viewer for a given portion of the E.M.S. The E.M.S.creation of the shadow (umbra) or the moon’s emergence out of the shadow is not affected: but it is the vision of the viewer that is affected.
Having said about the manner in which the visibility of the moon from a given point on earth changes let us turn to the question of measuring time with reference to the moon. New moon is the day the moon is ‘reborn’ that is the point of time when the ‘waning’ ends and ‘waxing’ begins. That is the end point where its non-visibility ends or where it emerges from the umbra or in other words alignment of sun, moon and the earth gets disturbed by their moving away once again going to different places. The movements of the EMS being steady the process of the shifting of the balance is continuous and smooth. It has no relevance to the point on earth from where one views nor is it dependent on the viewer’s position. Naturally the visibility depends on the relative position of the EMS. Because they move in different orbits, different planes and at different relative speed it is not always possible for a person stationed at a fixed point on earth to see the moon exactly at the moment it emerges out of the umbra or it is born. Due to the same reason the duration of visibility also gets affected.
The sun’s beams of light, in the manner observed above come from different angles and planes during various periods as the earth and the moon move in different planes and in different orbits.
There are two conditions, which enable our vision of the moon. The first is that the sun shall have set and the intensity of the sunlight – Halo (i.e. light remaining after the sun set) must be less than the brightness of the moon. Try to put on all the lights in your room and look out into darkness or less illuminated areas. Hardly anything can be seen. Reverse the position, go to the dark area and look into your room. Everything is very clearly seen. The same phenomenon applies here. Moon becomes visible when the sunlight or Halo is partially extinct. This takes about 12 minutes. The second condition is that the angle of visibility must be sufficiently wide for the viewer on earth to see the moon’s crescent and to realize that it is the moon. Theoretically a fraction of a second could be enough to see the moon but it will take a few seconds for our senses to register the same.
Though the moon may not be visible from a particular point on earth it will be visible from some other point, which satisfies these two conditions, intensity of light and angle of vision. Whatever be the position of the viewer the fact of the birth of the ‘new moon’ happens in the manner it should.
In conclusion it can be seen that new moon - Hilal occurs once every 29.53days and will appear at such points on earth, which fall in the angle of vision and will be visible at a time when the intensity of the halo (twilight) is less than the intensity of the Hilal. Whether visible to any person or not the Hilal must occur at regular intervals as the Sun, Moon and Earth move according to predetermined paths the crossing of which can be calculated as declared by the Qur’aan: “the sun and the moon are subject to calculation”.