Everyone will be looking to the sky on August 21 when a total solar eclipse will be visible in totality within a band across the entire contiguous United States.
The last time a total solar eclipse was visible across the entire contiguous United States was during the June 8, 1918 eclipse.
Solar eclipses happen when our view of the sun is blocked by the moon. When the moon lines up between the sun and Earth, the moon will cast a shadow onto Earth. This is what we on the ground observe as a solar eclipse.
We know when they’ll happen because over centuries astronomers have measured very precisely the motions of the Earth, moon and sun, including their orbital shapes, how the orbits precess and other parameters.
With those data about the moon - and similar information about the Earth’s orbit around the sun — we can make mathematical models of their movements in relation to each other.
Using those equations, we can calculate tables of data that can predict what we will see on Earth, depending on location, during an eclipse as well as when they will happen and how long they last. (The next major solar eclipses over the U.S. will be in 2023 and 2024.)
A solar eclipse happens, on average, a couple times a year. The moon passes between the Earth and sun every 29 days, a time we call the “new moon” - when the moon is not visible in Earth’s nighttime sky.
However, the moon’s orbit and the sun’s path in our sky don’t match up exactly, so at most of those new moon events, the moon appears above or below the sun.
Twice a year, though, there is a period where the moon and the sun line up with Earth — astronomers call this an eclipse season. It lasts about 34 days, long enough for the moon to complete a full orbit (and then some) of the Earth.
During each eclipse season, there are at least two eclipses visible from some parts of the Earth. At the full moon, there will be a lunar eclipse, when the moon passes directly behind the Earth, resulting in a darker, reddish-colored moon. And at the new moon, there will be a solar eclipse, when the sun is blocked by the moon.
Experts say that in the midst of the “Eclipse-mania” that those that wish to view this astronomical event do so with caution.
Counterfeit eclipse glasses have become a public safety health issue; deficient solar eclipse filters can quickly cause irreparable eye damage inasmuch as the retina has no pain receptors.
In particular, it’s not enough that the glasses have a logo and the ISO number 12312-2; customers must be reasonably confident that the glasses have come from a reputable manufacturer and/or authorized dealer
They can be quickly tested; however; a person should not be able to see much through a safe solar filter except for the Sun itself, sunlight reflected off shiny metal, the hot filament of unfrosted incandescent light bulbs, bright halogen light bulbs, bright LED flashlights, and an arc-welder’s torch. A welder’s helmet can be used for safe viewing provided it has a filter of 12 or higher.
It is never safe to look directly at the sun during the eclipse. Except during the very short period of totality do not look directly at the sun without approved solar-viewing devices.
Once the moon completely covers the sun, it is safe to look at the eclipse with your naked eyes.
As day briefly turns to night, stars and planets can become visible in the sky, temperatures noticeably drop, and your surroundings may become silent as animals react to the sudden darkness.
People outside the path of totality can use filtered glasses to watch the partial eclipse. And anyone can watch indirectly using a pinhole camera, which projects an image of the sun onto a flat surface for safe viewing.
