About every 18 months, the moon completely aligns itself with the surface of the sun and casts a narrow shadow on the earth's surface.
This is a total eclipse, and the next one is soon: Tuesday, July 2nd. On this day, a shadow will fall over the southern Pacific and parts of Chile and Argentina. It is the first total solar eclipse since the Great American Eclipse in August 2017, which divided the American mainland into two parts.
People fortunate enough to be on the path of totality will see a sublime sight (when it is not cloudy): The entire face of the sun becomes black from the moon, all surrounded by a brilliant white light. This bright white light is the solar corona or atmosphere. And it is only visible to our eyes during a solar eclipse.
The upcoming solar eclipse could be especially beautiful as it will take place just before sunset. The peak of the solar eclipse, the whole, takes about 2 minutes.
Here is the path that the entire solar eclipse will take in South America:
And here is an enlarged view of the Path of the Shadow. It will happen near Buenos Aires, the capital of Argentina.
Here is an artistic depiction of how the solar eclipse might look there.
South America is very far from me. Can I stream the eclipse live?
Probably the best place to observe the eclipse is the La Silla Observatory, an astronomy facility perched high on a mountain in the Chilean Atacama Desert. There begins the total eclipse, away from light pollution and relatively safe from the danger of a cloud cover, at 16:39 local time, just over an hour before sunset.
Chile is in the same time zone as the eastern United States. And many of us will be able to follow from a distance: The European Southern Observatory, which operates La Silla, will stream the event live on Tuesday, July 2, starting at 15:15 ET.
Why do we have sun? Eclipses?
The simple answer: Because the moon occasionally covers the sun on its way across the sky.
The complicated answer: Because of three key conditions that rarely all occur simultaneously.
1) That must be a new moon
One side of the moon is always illuminated by the sun, but the illuminated side does not always point to the earth. So we get the moon phases. For a solar eclipse to occur, it must be in its new moon phase.
During the New Moon, the dark side of the moon faces the earth directly.
2) The Moon Must Cross the Earth's Orbit The Moon must face the Earth for a solar eclipse to set in. Why do not we have her every new moon?
Because the moon's orbit is not perfectly in line with Earth's orbit.
It is tilted by 5 degrees as follows:
(No one is quite sure why – but maybe it has something to do with how The Moon probably originated: from a massive object smashed into the earth.)
Basically, this means that the shadow misses the earth during most new moons.
However, there are two points in the orbit of the moon to which the shadow may fall the earth. These are called nodes.
For a total solar eclipse to set in, the moon must be at or very close to one of the nodes.
3) The distance of the moon from the earth
Perhaps you remember from the science of middle school: The orbit of the moon around the earth is not a perfect circle. It is an ellipse.
There is a point in orbit where the moon is farthest from the sun and a point where it is closest to the sun. For a total solar eclipse to occur, the Moon must be near its closest approach to Earth.
Eclipses can occur when the Moon is at its peak (at the greatest distance from the Earth), but the Moon does not block the entire Sun. These are referred to as "ring-shaped" or "ring of fire" clips, as an incandescent sunstrip surrounds the dark face of the moon. You look cool.
If all three conditions match, you will get the following: The Earth and the Sun are aligned with the Moon at a node, and a shadow is cast on the Earth. (The opposite configuration – when the full moon faces the earth – is when we have lunar eclipses.)
The darkest part of the shadow is the whole – here the entire solar disk is darkened by the moon. While most of the US will experience a partial eclipse on August 21, only those in the shadow's 100-kilometer porthole will see the entirety.
When is the next solar eclipse in the US?
Approximately every 18 months, total solar eclipses occur somewhere in the world . That's how long it takes for the specific conditions leading to eclipses (the lunar phases, the distance of the moon to the earth, and the moon crossing the Earth's orbit) to be in line again.
The next will take place on December 14, 2020. It will also go through Chile and Argentina (luckily!).
The next solar eclipse over the United States will be in 2024.
After that? 2045. Then in 2052, 2078, and, if you are truly blessed with longevity, in 2079, a great one over Maine. That is, mind you, five millennia.
Therefore, for example, we know that on January 27, 2837 total solar eclipse will pass through southern Mexico. (Will anyone be there to see it?)
It is reassuring to know that in 1000 years people can look in wonder at the same natural phenomenon that we see today. Life on earth may change, but sun, moon and sky are not.