The Moon has served important functions throughout history, both for humanity and for planet Earth. And, for the Muslim world, it continues to play an important role in determining holy occasions: Ramadan, Eids and Hajj, most notably. The Moon, also known as Luna, revolves around Earth (in a month) and, depending on its location with respect to our planet and to the Sun, it reflects varying amounts of light and thus appears in “phases” or shapes that go from thin crescent to full moon and back. Those phases tell us (roughly) which night of the lunar month it is. They also help us to determine directions, on land and at sea, which can be used for simple navigational purposes. The Moon is the simplest universal visual calendar one could have, and that is why it was adopted as such from the earliest times of human history. It was later replaced by the solar calendar by most cultures; because the lunar year is shorter than the solar year and thus shifts through seasons, it does not coincide with agricultural periods (of planting and harvesting) or weather conditions (rainy and hot seasons), in particular. The fact that the Moon very easily tells us the day/night of the month is the reason why it was adopted by Islam as the indicator of the start and end of each month, particularly Ramadan, as people need to know when to start and when to stop fasting. With scientific progress, we have realized that this method has its own shortcomings, but astronomy and computers allow us to compensate for these drawbacks, as Muslim astronomers (like me) have been stressing. But the Moon has played other vital roles in the history of our planet. Luna is one of the largest moons (objects that revolve around a planet) in the solar system; in fact, when we compare each moon to its parent planet, ours is by far the largest. And thus it has important physical effects on the planet, and consequently on animals and life; in fact, the Moon played a crucial role in the early evolution of life on earth. The most important effect that the Moon has on Earth is the tides: The sea water rising up and falling back on beaches every day. This leads to the mixing of chemical compounds and biological organisms between the sea and the land, allowing for the evolution of plants and animals early in the history of life on our planet. Tides are also the reason why the rotation of the Earth has been slowing down (at a rate of two milliseconds per century, or half an hour every 100 million years). Our planet rotated around itself in about six hours when the Moon formed, but the rotation period is 24 hours today and it will be 26, 28, 30 hours in a few hundred million years. This phenomenon also leads to the gradual migration of the Moon away from the earth, by 3.8 centimeters per year, so it used to be much closer to Earth. It has been vital for us as humans, serving as a visible calendar and as a marker of religious dates, as well as for life on Earth and the planet as a whole. Nidhal Guessoum And, last but not least, the moon produces regular solar eclipses. Eclipses are not only stunning phenomena (and Earth is the only place in the solar system where total solar eclipses can be seen), they have allowed us to learn a number of things about the sun and the cosmos. Some years ago, Neil Comins published a book titled “What If The Moon Didn’t Exist,” in which he described a very strange planet Earth: Fast rotation (very short days), very strong winds preventing palm trees (for example) from growing, very slow evolution of life, very weak tides (produced by the Sun), no eclipses, and no nocturnal life (many animals rely on the Moon for light and navigation). As we come to the end of Ramadan and Muslims look for the crescent to determine when Eid will fall (astronomers like me can tell in advance that the crescent will be seen on Thursday, June 14, and thus Eid will be on Friday, June 15), let us spare a thought for this wonderful celestial object, our Moon. It has been vital for us as humans, serving as a visible calendar and as a marker of religious dates, as well as for life on Earth and the planet as a whole. Luna has also been a favorite motif of poets. For example, the great Sufi master and poet Jalal ad-Din Rumi said: “Who could be so lucky? Who comes to a lake for water and sees the reflection of the Moon?” And: “At night, I open the window and ask the Moon to come and press its face against mine. Breathe into me. Close the language-door and open the love-window. The Moon won’t use the door, only the window.” Eid Mubarak. Nidhal Guessoum is a professor of physics and astronomy at the American University of Sharjah, UAE. You can follow him on Twitter at: www.twitter.com/@NidhalGuessoum.