What's the difference between eclipses and a new moon?

As I understand from http://blogstronomy.blogspot.com/2010/01/whats-difference-between-new-moon-and.html#!/2010/01/whats-difference-between-new-moon-and.html, a lunar eclipse is:
This normally results in a full moon, but when the moon happens to be in Earth’s shadow, it becomes a lunar eclipse. Does this mean lunar eclipses only occur during full moons? This I can get.

According to it, a new moon is:

  1. Isn’t this a solar eclipse?
  2. How can we see new moons then? In this position the only parts of Earth that can see the new moon are experiencing day.

Yes, a solar eclipse is the newest moon possible. The entire sunlit face of the moon is facing away from the earth.

There is nothing special about the time of day that you see the moon. You can see the moon through most phases anytime of the day or night that it is above the horizon. It is however rather poor contrast during the day, the blue sky competes with the low surface brightness of the moon, and many people simply don’t notice it. When the moon is close to the sun during the day it becomes harder to see, but it is visible. Not until it gets really close to the sun does it become essentially impossible to see.

Yes, but most of the time the three bodies aren’t lined up as directly as your description implies. A normal new moon (sans eclipse) looks like.


(not even close to scale) where the moon is “between the sun and earth” in some sens, but not directly on the line between the two. Of course, if the moon is too far out of line, we are going to perceive it as different phase, but there is some slack here.

Again, the moon need not be directly in line, so it may appear at night just before sunrise and just after sunset when the sun is just below the horizon, but it is not.

A solar eclips will happen only when the moon is directly between the earth and the sun, with all or some of the suns shadow hitting the earth. Normally, the moon does not actually block any of the sun, as seen from the earth, during a new moon.

The Moon’s orbit around the Earth is not in exactly the same plane as that of the Earth’s around the Sun. Therefore, on most of the Moon’s orbits around the Earth, it is either above or below a line directly between Earth and the Sun. Only occasionally do the orbits intersect in such a way that the Moon is directly between the Earth and the Sun so that the Moon blocks the Sun. This is a solar eclipse, which only happens when the Moon is in its “new” phase.

We don’t normally see the Moon when it is completely new. None of the Moon’s surface that is facing toward us is sunlit, and in any case it’s up entirely during the day. However, once the Moon is a day or two away from new a narrow crescent will be illuminated that may be visible even in daylight.

Even the completely new moon can be seen, if you look for it, since even the “dark” side will still be illuminated by Earthshine (light from the Sun that reflects off the Earth and illuminates the Moon). Right at new moon exactly, this would be difficult to see if only because you’d have to pick it out from the glare of the Sun, which would be quite close in the sky. But a day or so before or after, you can often see “the new moon in the arms of the old”: A thin bright sliver lit directly by sunlight, with a dimmer but still visible portion completing the visible disk, from Earthshine.

Yes. Lunar eclipses can only happen at full moon, and solar eclipses can only happen at new moon.

Nope, not with the naked eye. The Muslim calendar is based on months that begin when the new moon is first seen, and they look for it VERY carefully. Nobody credible has even claimed to have seen it within 12 hours of the actual, bang-on new moon, and claims of less than 15 hours are treated with extreme skepticism. Depending on the alignment, it may take a full day.


Yes, but where are those observations made? Are any of them from, say, the top of Mauna Kea? Get above enough of the atmosphere, and block the Sun with some sort of shade, and it should be quite possible.

Ah thanks, this made it a lot clearer. Thanks to everyone who replied as well. I always thought new moons were caused by the same way as lunar eclipses, which was confusing.

I think this qualifies as “not normally seen.”

Warning: Technobabble ahead. Proceed at your own risk

The imaginary line drawn from the center of the Earth to the center of the Sun, and projected against the backdrop of the Stars is called the “Ecliptic”, because, when an eclipse occurs, the Moon must be at some point on this line in alignment with the Earth and Sun. The Ecliptic passes through the 12 signs of the zodiac.

The Earth’s equator, when projected against the backdrop of the stars defines the “Celestial Equator.” The Earth’s rotational axis is tilted by about 23½ degrees from the plane of the ecliptic, and, therefore, so is the Celestial Equator.

The plane of the Moon’s orbit is tilted by about 5 degrees from the ecliptic, in contrast to most other moons in our Solar system which (other than captured asteroids and such) orbit within a few degrees of the primary’s equator (cf. Jupiter and Saturn.)

Due to an amazing cosmic coincidence, the angular extent of both the Moon and the Sun, as seen from the surface of the Earth, are almost exactly equal, and about ½ degree. Both may vary a bit, as the orbits of neither the Earth nor the Moon are perfect circles, and as we get closer or farther from the Sun, its apparent diameter grows or shrinks (by a very small amount.) The same with the Moon.

As the Earth-Moon system orbits the Sun, occasionally the Moon passes directly between the Earth and Sun, causing a solar eclipse. But, since the center-to-center distance between the Moon and Sun (as viewed from the surface of the Earth) can be as much as ±5 degrees, and each is only ½ degree, more often than not the Moon will pass far “above” or “below” the Sun. Sometimes, when we are nearer than usual to the Sun (making it appear slightly larger) and the Moon is farther than usual (making it appear slightly smaller), when the two are aligned center-to-center, the Moon does not completely block the disk of the Sun, and a thin ring, or “Annulus” (Latin for “ring”) of Sun shows around the silhouette of the Moon. Annular eclipses are slightly more common than total solar eclipses.

At the average distance from the Earth that the Moon orbits, the central, darkest part of the Earth’s shadow (the “Umbra”) is a few degrees wide. Most months, at the time of a full Moon, the Moon is passing well above or below the Earth’s shadow. On average, about twice a year the Moon passes into the shadow, but not always into the central umbra, and at these times there is a “Penumbral” eclipse (pen (from Latin paene, near) + *umbra *(Latin for shadow)).

The Earth’s rotational axis is tilted by about 23½ degrees from ***the normal to ***the plane of the ecliptic.


Wow I didn’t know the Ecliptic passes through the zodiac. Are the tilts of the Earth’s rotation and the moon’s orbit in the same direction?

The normal to the plane of the Moon’s orbitis not currently “in the same direction” as the Earth’s axis of rotation, but, like the Earth’s axis “precesses,” so does the Moon’s orbit, due to perturbations of the Sun and Earth. At some time in the past, and again at some time in the future, the two will likely point in more or less the same direction.

Couldn’t one ‘see’ a completely new moon by watching the occultation of stars behind it?

Since a new moon requires that the moon and the sun be on the same side of the Earth, a new moon can only be seen during the day. No stars, other than the sun, are visible.

Not unless you’re in space, since if it’s a new moon to begin with, it’s very close to the Sun in the sky, and so the stars would be washed out by daylight. Seeing the Earthshine would be much easier.

EDIT: Actually, I suppose you could say that, technically, watching a solar eclipse is “seeing the new moon by watching the occultation of a star behind it”.

The Ecliptic actually traditionally defines what constellations are in the Zodiac, since these are (mostly*) the constellations that can appear as a backdrop** to the Sun (and Moon and planets, since they are mostly in the same plane).

*The Ecliptic actually crosses Ophuichus as well as the traditional ones, making 13 instead of 12 Zodiac constellations. And the planets (not including Pluto) can appear in at least 21 astronomical constellations.

**Well, they’re not actually visible when the Sun is in them, since then they’re up during the day.

A well-deserved “Duh” for me. Thanks.