How do astronomers/scientists determine the phases of the Moon like the occurrence of the New Moon?

Question

How exactly does the USA Naval Observatory predict when the New Moon occurs? Is it 100% accurate? Is it 95% accurate? Is there room for error?

http://aa.usno.navy.mil/data/

Answer 1

The position of the moon can be measured by telescopes with precisely calibrated mounts. These have been the standard astrometric instrument since the 17th century. These days, they also use laser beams, bounced off reflectors left on the moon by the Apollo program, to precisely measure the distance to the moon. The most accurate distance measurements are good to within a millimeter.

According to JPL, current ephemerides can predict the position of the moon to an accuracy of about 1 meter. That translates into a time accuracy of about a millisecond.

Answer 2

The phases of the Moon (i.e. new Moon, full Moon, etc) are extremely well known and have been accurately charted by astronomers for thousands of years. The cause of the Moon's phases is the fact that as the Moon travels around the Earth, the angle between the Earth, the Moon and the Sun changes. So when the Moon is roughly on the opposite side of the Earth as the Sun is, the entire part we can see is reflecting light from the Sun, and so we have a full Moon. And when it is on the same side of the Earth as the Sun is, only the side we can't see is being lit up by the Sun, so our side is dark, giving us a new Moon. The periods of the orbits of the Moon and the Earth are very consistent (about one year for the Earth, about 28 days for the Moon) and have remained virtually unchanged throughout all of human history. Simply by knowing the current positions of the Earth and the Moon in their orbits, scientists (or any other intelligent person, for that matter) can easily determine the date of the next new Moon, full Moon or any other phase with a high degree of accuracy. The predictions of astronomers on the phases of the Moon are therefore both extremely precise and extremely accurate, and to throw their predicted timing off by more than a few seconds would require some very large astronomical event, such as a collision between the Moon and some other very large space object, but such an event is extremely unlikely to happen anytime soon and if it did we would have other things to worry about (like the potential for debris to impact the Earth).

>An answer like "scientists can precisely measure the Moon's orbital position" is not the answer I'm looking for. I'm asking HOW can a scientist know when the New Moon will occur next year ?

But that IS how they do it. Knowing the current positions of the Moon and the Earth and their orbital directions and periods, simple arithmetic will allow one to predict the phases of the Moon for many years into the future (with the time extended proportionally to the accuracy of the known information). For example, if it is currently a new Moon, then the next full Moon will occur in about 14 days and 18 hours, because that is half of the Moon's phase period of about 29 days and 12 hours (the actual value is not precisely 29 days and 12 hours but that is a close enough estimate for this example). Adding this value again shows that the next full Moon after that will arrive in about 44 days and 6 hours, and the next one after THAT in 73 days and 18 hours, measuring from the present moment. You can add other amounts of time to determine the point at which other phases will occur.

>If you answer the question and have any qualifications, please post them (like if you have any degree or experience in Astronomy/Astrophysics)

I don't have any qualifications other than my top contributer label, but this is really rather simple astronomy and any educated person should be aware of the mechanics of the Earth, the Sun and the Moon and how they determine the phase of the Moon.

Answer 3

This knowledge has been accurately calculated even before humans understood the motions of the earth, sun and moon.  

Thanks to a great deal of advancement in the field of physics in the seventeenth and eighteenth centuries, we have been able to determine the orbital speed of the moon, even the varying speeds based on its perigee and apogee.  These calcualtions are presice and accurate.  The only thing that could alter these calculations would be the prescence of a nearby massive object (a large asteroid, perhaps) suddenly getting close enough to us, perturbing the motions of earth and/or the moon due to gravity.  This object would have to be much more massive than most of the asteroids that have any orbits anywhere near earths, so if it were to happen, it would be detectable long before it's effects could be noticed, and nasa, or any legitimate student of physics, would be able to make adjustments on the new moon phase several months in advance, just by plugging in the new variable into the formula.

Answer 4

From the USNO? Absolutely 100% accurate. Its based on observational data going back hundreds of years.

Anyone can get the necessary nautical almanacs and compute all phases of the Sun, moon and stars. You don't need to be an astronomer,just how to do celestial navigation.

Answer 5

The orbit of the moon is very precisely known and measured.  Scientists know to the second when new moon, full moon, and other phases occur.  That site is as accurate as any other site.

Answer 6

Calculations on the position of the Moon, and hence timing of phases, eclipses, conjunctions, syzygies, etc are based on something called lunar theory, which has a very long mathematical history, with considerable input over the past three centuries from several different mathematical astronomers.  For some introductory material on it see either of the two books on eclipses, one for lunar and one for solar, issued by NASA and authored by the team of Fred Espenak (NASA's in house expert on eclipses) and Jean Meeus (a Belgian astronomer famous for his studies of positional astronomy).  This is a very technical area of astronomy involving some complex math, as it involves allowing for Earth's equatorial bulge, gravitational effects of a couple of other planets as well as the Sun, etc.

When the Apollo Project was first being planned in 1963 the distance to the Moon was not known to an accuracy of better than two kilometers.  With the placement of several laser reflectors on the Moon, its position is now defined with an accuracy of one centimeter, and its motions are quite well known and understood for the most part.  The mere fact that eclipses are predicted to the second shows how precisely these things are known today.

The Espenak/Meeus book on solar eclipses predicts them through the year 3000.