Science of moon-sighting

This angular separation reflects the sun’s light to earth. Until this angle becomes seven degrees, no light from the sun reflected by the moon can come to the earth, because of the mountains on the surface of the moon that block the sunlight (Danjon 1932). This angle must be about 10-12 degrees from the sun’s […]

This angular separation reflects the sun’s light to earth. Until this angle becomes seven degrees, no light from the sun reflected by the moon can come to the earth, because of the mountains on the surface of the moon that block the sunlight (Danjon 1932). This angle must be about 10-12 degrees from the sun’s light to reflect from the moon making a thin crescent that can be sighted.

Another important factor is the moon’s altitude above horizon. If the crescent’s thickness is sufficient, but is not above the horizon, then it cannot be seen. This happens in the months of March and April in Nigeria when the moon is in the northern hemisphere; therefore, it cannot be seen from the southern hemisphere. The moon within the altitude of 10 degrees at sunset is usually not visible.

It is also a scientific fact that if the moon is visible anywhere on the globe, it would be more easily visible on all places west of it. On the contrary, we hear the news of moon-sighting in the Middle East, and on the same evening here in West Africa hours later, it is not visible despite clear horizons. This clearly tells us that what was seen in the Middle East could not have been the moon.

Muslim astronomers in this age of technology are now in a position to calculate the dates for possible/impossible moon-sighting. So the validity of sighting claims can easily be verified for authentic moon-sighting. To start an Islamic month correctly, we must examine sighting claims in light of the knowledge that Allah (SWT) has given us. Let us pray to Allah (SWT) to open our hearts and minds to understand these facts about moon-sighting.

However, there are several misconceptions about moon-sighting. The same are discussed below:

Misconception 1: The 29th-day moon is always thin and 30th-day moon is thick. It is very possible that the 30th-day moon of one month is about the same thickness as the 29th-day moon of another month. The moon revolves around the earth in an elliptical orbit; therefore, sometimes it is closer to the earth and sometimes farther. If the moon is 20 hours old on the 29th day of one month when it is closer to the earth, it may look about the same size as when it is 25 hours old on the 30th day of another month and is farther from earth.

Misconception 2: The 29th-day moon sets in a short time while the 30th-day moon remains longer on the horizon. The moon above the horizon, an hour before setting, travels down the horizon line along a path whose inclination depends upon the latitude of the observer. A vertically-downward path would be an equator, and a 45-degree slope at 45 degrees latitude. The moon that is 10 degrees above the horizon at the equator would set quicker than a moon that is 10 degrees above the horizon at latitude of 45 degrees. Therefore, it is not correct to say that the 30th-day moon remains longer on the horizon. Some months it may be, but not always.

Misconception 3: If the moon is thick and remains on the horizon a long time, it must be the 2nd-day moon. This is a very common argument that we hear repeatedly, but has no truth in it. If on the 29th day, the moon at sunset is 18 hours old and is higher above the glare of the sun, it may be visible on that day and will appear to be very thin. However, if the 18-hour-old moon on the 29th day is very low on the horizon, it would not be visible on that day; then on the 30th day, it would be 42 hours old and quite thick and remains above the horizon a long time, but it was not visible the day before.

Misconception 4: On new-moon day, the dark side of the moon faces the earth. By this sentence, people think that the far side of the moon is the dark side, and that dark side faces us at new-moon day. That is not the case. The same side of the moon always faces the earth, sometimes illuminated and sometimes dark.

The moon takes 27 days, seven hours and 43 minutes to spin or rotate once on its axis. It takes the same amount of time to orbit the earth. The combination of the two motions keeps the same side of the moon facing the earth. To us on earth, the moon is fully illuminated at full moon, completely dark at new moon and partially lit in between.

Misconception 5: Three consecutive months of 30 days are not possible. Three consecutive months of 30 days are very possible and are not uncommon. In fact, even four consecutive months of 30 days is possible, but rare. This is mainly due to considering the whole West Africa as one horizon. Because of the elliptical orbit of the moon, two consecutive months of 30 days are quite common.

In the third month, on the 30th day of the previous month, sometimes the moon is only sighted on the west coast, because for the east coast, it may have been the 29th day from the actual sighting in the east coast. If that is the case, then in the fourth month, the moon may take 29 or 30 days to be visible. In the latter case, it would be four months in a row that make 30 days’ month.

The Committee for Crescent Observation Centre for Basic Space Science and Astronomy, University of Nigeria, Nsukka has been keeping records of sightings in Nigeria for the last two years and confirms that three consecutive months of 29 days and four consecutive months of 30 days have occurred several times in Nigeria.

Misconception 6: Full moon occurs on the 14th night. This is wrong too. Full moon is the instance of time when the moon crosses an imaginary line passing through the centres of the sun and earth. This could happen at any moment of the day or night. Therefore, it is very possible that in one month, the moon is full at 7:00 a.m. This moon would be 99% lit on the night before and becomes full in the morning, but decreases to 99% lit-up on the following night. Such a moon would appear as a full moon to an observer on both nights. These two nights could be 13th and 14th or 14th and 15th depending upon the location on earth and the time of the new moon birth.

Aliyu is of National Space Research and Development Agency, Enugu.