A Scientific Overview Of Eid Moon Sightings

Sightings of the new crescent moon have been used for millennia to signal the start of many religious festivals and celebrations. They are still very important today for local communities but suffer with large global inconsistencies which can have wider reaching effects on business. This is especially true when countries disagree on the day of the new moon. One of the most discussed and disagreed upon moon sightings relates to the start (Hilāl) and end (Eid al-Fitr) of Ramadan. Many communities still prefer to conduct their own local moon sightings to signal either the beginning or end of Ramadan. Therefore we look at the science behind moon sightings and what this might mean at a global level.

What is a new moon?

A new moon is when the surface of the moon facing Earth is pointing directly away from the Sun (see the image below). At this configuration no sunlight is reflecting off the surface of the Moon facing towards Earth. Incidentally if the moon happens to be aligned perfectly with the Sun a new moon will also coincide with a solar eclipse.  However, many additional things mean that solar eclipses occur a very small fraction of all the new moons we see.

lunar phase

The start of a new lunar month begins when the first light from the crescent moon is observed. As there is no sunlight reflecting towards Earth at the point of the new moon we have to wait until the moon has moved further 7 degrees on its orbit to make a sighting. Typically this occurs after it completes a further  of its orbit and is why sightings are not made until 11 – 15 hours after the true new moon.

Interestingly though there is enough sunlight reflecting off the surface of the Earth to illuminate the Moon, known as Earthshine. Depending on seeing conditions the shadowed area of the moon can still be seen naked eye due to this effect.

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Why Can’t The New Moon Be Viewed By Everyone At The Same Time?

The new moon occurs approximately at the same time regardless of where on the planet you are located. However, the main issue arises due to its visibility. We are trying to view an astronomical event from various locations on the surface of a sphere (Earth). Naturally there are many locations where the event cannot be observed from Earth.

visibility 1

An online resource which creates visibility maps (shown below) of the new moon can be found here. Next we are going to discuss the visibility and why it is constantly changing with respect to fixed locations on Earth.

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Sidereal and Synodic Period:

Firstly something that needs to be considered is the orbital period of the Moon. Typically we talk about two different periods, Synodic and Sidereal. The sidereal orbital period is the time it takes the Moon to do one full orbit around the Earth, so 360 degrees relative to fixed stars. The sidereal period is then given as 27.3 days. The sidereal period does not relate to the lunar phases though, instead this is linked to the synodic period. The time from one new Moon to the next is longer at 29.5 days. The reason for this is the lunar phase originates from the alignment between the Sun, Earth and Moon. Since the Earth is also orbiting the Sun the Moon has to complete more than one full orbit to reach the same Sun-Earth-Moon alignment. This works out at a period of 29.5 days with the Moon travelling approximately  390 degrees around the Earth to reach the same alignment.

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The Inclined Orbit Of The Moon:

The fact that we observe a new moon when the Moon is at a different part of its orbit each time would be fine if its orbit was aligned to rotation axis of the Earth. Unfortunately for the case of new moon visibility this is not the case. The orbit of the moon around the Earth is inclined by 5.14 degrees. So taking into account that a new moon will occur at a different part of its orbit each time and could occur at any angle between +5.14 and -5.14 degrees from the equator. Couple this with the tilt of Earth’s rotation axis and the visibility of the Moon in the local sky during the year changes significantly. The illustration below depicts the viewing angle of a full moon during winter and summer from a latitude of +35 degrees. During winter the full moon would be close to directly above at midnight where as during the summer it would be closer to the horizon.

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Some Other Considerations:

There are some other factors which will also alter the visibility of new moon depending on the location of the planet you are. Again these factors will be at a slightly different position or angle in the local sky at the time of the new moon. The first is the tilt of Earth’s rotation axis actually makes out a circle with respect to fixed stars with a period of 26,000 years. It can be thought of as a spinning top that slowly makes out a circle as it is spinning much faster. It is an easy experiment to carry out for yourself. The axis that the Earth spins on wobbles in a similar manner but with a period of 26,000 years and is known as axial precession. Another factor is that the tilt angle itself which varies between 21.5 – 24.5 degrees with a period of 41,000 years. Again this is going to change the viewing in the sky of any new moon when making local observations.

tilt2

The orbit of the Moon also undergoes precession. So an elliptical orbit like the Moon will rotate through 360 degrees. This is known as Apsidal Precession and for the Moon has a period of 8.85 years. It should be noted that these are long term factors which will change over the course of years and thousands of years.

Final Thought

Due to the fact that sometimes the new moon is not visible from all locations an incorrect day could be assigned to the start and end of Ramadan. If local observations have to wait until the moon is visible then it is likely that in these cases the moon is already on its 2nd day of the lunar month and sometimes even the 3rd. The question is if observations of the new moon are not possible then what is the best approach for deciding on the day of a new moon sighting?

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