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How To Sight The New Moon Crescent?
Sighting of the Shawwal crescent,is an exciting event adding more joy to the Eid celebrations. This has changed in recent years, and various controversies over the sighting of the new crescent have spoiled the tradition. Here, it would not be a bad idea to take help of scientific knowledge of the celestial events in order to predict new crescent.
Methods to predict the new crescents have been in use since ancient times, perhaps even before the Babylonians. However, it was not until the 20th century that the western world began substantial work on the subject since they had little interest in this regard. But very few people are aware that the great Muslim scientist, Abu Rehan Al-Biruni developed an extremely precise method of predicting the sighting of the new moon, more than a thousand years ago.
In order to understand how it is possible to predict the visibility of the crescent, let us consider some basic concepts. Unlike the full moon that travels across the middle of the sky all through the night, the new moon appears in the western sky where the sun sets. Its path makes a small arc and it disappears below the western horizon after a few minutes. The sun lies towards the opposite side of the moon during this period when seen from the Earth, so that the un-illuminated hemisphere of the moon faces the Earth, and hence the moon largely remains invisible to us. However, after a brief period of complete invisibility (the conjunction of the sun and the moon), a little crescent of illuminated portion can be seen on the margin of the moon, and this is regarded as the thin crescent or ‘hilal'.
The process occurs because the sun shifts a little off the Earth-moon axis (Fig. 1). When the entire hemisphere of the moon facing the Earth becomes totally dark, the event is termed as ‘the conjunction of sun and moon' or ‘birth of the new moon'. This event occurs 18 to 24 hours before the crescent becomes visible to us.
At the end of the lunar month, if we look towards the sunset in the western horizon, the sun and the moon can be in any of the following three positions relative to each other and the Earth's horizon.
The sunset has occurred while the moon is still in the visible horizon (Fig. 2). This is the time when the new moon can be seen, provided the weather is clear.
The moon has disappeared below the horizon while the sun is still above the horizon (Fig. 3). Here, the crescent is impossible to be seen.
The sun and the moon are both above the horizon simultaneously (Fig. 4).The naked eye cannot see the crescent due to brightness. However, the crescent can be seen with the help of powerful telescopes.
The position of the celestial bodies is determined by their altitude and azimuth, just as a location on Earth's surface can be determined by its latitude and longitude. These positions of the heavenly bodies can be, and have been calculated for several hundred years. Celestial events such as the solar and the lunar eclipses are predicted using these calculations.
Keeping this in mind, one may think that if the azimuth and altitude of the sun and the moon are known for a certain location on Earth at a certain date at sunset, then the arrangement of the sun and the moon relative to the horizon (position A, B, or C can) be predicted. Hence, the visibility of the new moon can be calculated. But this is not as easy: apart from the simple mathematics, several other factors, such as the refraction and diffraction of light and different states of atmospheric pollution contribute to the visibility of the crescent. Also, the limitations of the human eye in seeing small objects at a certain distance - the visual acuity - and the necessity of a certain minimal difference in the brightness of an object and its surroundings - contrast sensitivity - can interfere with the visibility.
All these phenomena create a small area (stretching across the globe as a strip or zone) of doubtful predictions. For the rest of the globe (more than ninety-five per cent), the visibility or non-visibility can be precisely calculated. Even for the zone of doubt, more accurate predictions can be made empirically, based on the statistical data of a large number of actual sightings.
Now let us consider Al-Biruni's method of predicting the new moon. But remember that this is a rather over-simplified description of a method for calculating the first sighting of the new moon, at a given location on the required date by means of a graph, (Fig 5). Following are the steps to calculate the prediction.
Find out the exact latitude and longitude of the place from where the crescent is to be sighted. Any atlas or some websites can provide the information. Some of the websites are that give this information are:
Find out the time of sunset of the location at the required date. There are many websites that can provide the information like,timeanddate.
Determine the azimuths of the sun and the moon at sunset. Altitude of the moon at this time is also needed. Acquiring this information from H M Nautical Almanac Office in London used to be a difficult process, but now anyone can get this information easily from the Australian Government's astronomical information website:
Find the algebraic azimuth difference between the sun and the moon (sun azimuth minus the moon azimuth), and plot it on the horizontal scale of the graph. (Fig 5)
Plot the altitude of the moon on the vertical scale of the graph. (Fig 5)
If the plotted point lies below the visibility curve, the crescent will not be visible, and if it lies above the curve the crescent can then be sighted, provided the weather is clear. If the point lies on, or extremely near the visibility curve, the crescent may or may not be visible due to the atmospheric factors and the human eye limitations, as discussed earlier.
Points plotted for several locations on the Earth on a given date can be used to draw parabolic curves across the world map, by which anyone can tell at a glance where in the world the new moon will be visible or, more importantly, not visible at a certain date.