It’s all about distance
The apparent size of the Moon with respect to the Sun varies considerably from one solar eclipse to the next, mainly because the Earth-Moon distance is always changing. Over the course of the month, it varies between 357,000 and 406,000 kilometres, because of the elliptical shape of the Moon’s orbit around Earth. Also, Earth’s orbit around the Sun isn’t a perfect circle either: over the course of the year, it varies between 147.1 million km (early January) and 152.1 million km (early July). The Sun thus appears to us a bit larger in winter than in summer.
During a Solar eclipse, when the Moon is closer to Earth (Moon larger) or the Sun is farther away (Sun smaller), the Moon’s umbra may touch the surface of the Earth. If we’re within the ground track of the umbra, the Moon completely covers the dazzling surface of our star. We witness a total eclipse of the Sun.
When the Moon is farther away from Earth (Moon smaller) or the Sun is closer (Sun larger), the Moon’s umbral cone falls short of reaching the surface of the Earth. But beyond the tip of the cone extends what’s called the antumbra, which does reach Earth’s surface. Viewed from anywhere within the strip of land swept by the antumbra, a ring of sunlight remains visible around the dark silhouette of the Moon : it does not manage to completely hide the Sun. This is an annular eclipse of the Sun.
Who sees what?
A total or annular solar eclipse is therefore visible to only a small fraction of Earth’s inhabitants. It is seen only by those who are within the narrow path swept by the umbra or antumbra of the Moon. That’s why eclipses are so rare!
As the Moon advances in front of the Sun, it blocks out an increasingly larger portion of our star, and the ambient luminosity decreases accordingly. These are the partial phases of the eclipse, and the Sun appears like a thinner and thinner crescent.
When the Sun changes shape or disappears
If we’re inside the path of annularity, an annular eclipse culminates with the annular phase, or annularity, when the silhouette of the Moon detaches completely from the rim of the Sun. For a few minutes (theoretical maximum is 12 min 30 sec), the Sun looks like a ring of fire.
In the case of a total solar eclipse, if we’re within the path of totality, the Moon eventually blocks out the bright surface of the Sun completely. This is totality: for up to 7 min 31 sec (theoretical maximum), the sky becomes dark, except for the horizon which takes on the colours of twilight. During this time, the brightest stars and planets appear and the delicate solar corona reveals itself around the black silhouette of the Moon. A stunning spectacle!
On either side of the path of totality or annularity, we find two, much larger zones swept by the Moon’s penumbra. From there, one only gets to see a partial eclipse to some degree: it doesn’t become total or annular.