The Earth’s orbit carries it around the Sun once a year – in 365.24 days, like clockwork. In the meantime, every 27 days, 7 hours, and 43 minutes, the Moon completes a single orbit around the Earth. This is where we get the term “month” from, and the two reliable and regular orbits are the bases for our calendars.
At the moment in the Moon’s orbit when it crosses the straight line between the Earth and the Sun, a New Moon occurs. Within a day or so on either side of the moment of New Moon, we can’t see the Moon within the Sun’s glare and, in any case, the half of the Moon pointing towards the Sun is fully lit, while the half facing us is dark. Roughly two weeks later, the Moon has travelled half of its orbit to place the Earth in the middle of the trio. This time, the Earth crosses the straight line between Sun and Moon, creating a Full Moon. At that moment, the entire side of the Moon facing Earth is illuminated by Sunlight, and the Full Moon rises just as the Sun sets at our backs.
If the Sun, Earth, and Moon were all travelling in the same plane (imagine a model of them on a tabletop), every New Moon would cross over the Sun and cast a circular shadow on the Earth, producing a Total Solar Eclipse visible by people inside the shadowed area. At every Full Moon the Earth would hide the Sun from the Moon as it passed through our shadow for a few hours, producing a Total Lunar Eclipse. So why do these events seem to be so rare?
The main reason eclipses occurs rarely, and are visible from different locations on Earth, is because the Moon’s orbit is tilted (or Inclined) by about 5° with respect to the Earth’s orbit around the Sun. While the bodies in the Solar System orbit the Sun in roughly the same plane, each body’s orbit is tilted by a small angle that carries it sometimes above and sometimes below the Solar System’s “tabletop”, like the horses on a revolving carousel.
On infrequent, but predictable occasions, the moments of New Moon or Full Moon occur when the Moon is situated in the place where its orbit and the Earth’s orbit intersect, also known as a node. This is the geometry that creates eclipses. The Earth has a much larger diameter than the Moon, so an eclipse can still occur when the Moon is below or above the node. That is why eclipses are visible from different parts of the Earth and why the tracks of totality change in both latitude and longitude.
By cosmic coincidence, the size of the Sun’s disk and the Moon’s disk observed from Earth are almost exactly the same, even though the Sun is about 400 times larger! If this were not so, we would not be treated to the splendor of a Total Solar Eclipse.
During a Total Solar Eclipse, the Moon’s round shadow sweeps across the Earth, completely blocking the Sun for observers along the track of the shadow for only a few minutes. The best eclipses have 250 km wide tracks and yield many minutes of totality. In zones alongside the track, observers see only part of the Sun obscured – the farther from the path of totality, the less of a “bite” is taken out of the Sun. This is called a Partial Solar Eclipse.
It is NEVER safe to directly look at a Solar Eclipse, with one exception. Observers experiencing the few minutes of totality may turn their gaze upon the Moon-obscured Sun and see the glorious corona. Everywhere outside of totality, some of the Sun’s disk is exposed, and any amount of unprotected viewing is harmful to your eyes. The next nearby Total Solar Eclipse will cross the continental USA on August 21, 2017.
In a Lunar Eclipse, totality lasts several hours because the Earth is larger than the Moon and casts a shadow that is much wider than the Moon’s. The Moon in totality tends to turn a ruddy red, orange, or pinkish colour because all the sunlight reaching it has travelled over the Earth’s horizon, scattered by our atmosphere. This is the same light that colours the landscape (and us) during sunrises and sunsets. It also means that some sunlight will always get to the Moon and prevent it from going completely dark.
Due to the Earth’s atmosphere, the shadow cast by our planet has a dark circular core called the Umbra surrounded by a ring of partial darkness called the Penumbra. As the Moon traverses these zones, we get eclipse phases known as Penumbral and Umbral. In a Total Lunar Eclipse, the Moon can cross the penumbra twice.
Since Full Moons are already 100% safe to look at with unaided eyes or a telescope, a Full Moon during Lunar Eclipse is just as safe to view.