Celestial Geometry: Equinoxes and Eclipses
March 20 marks the spring equinox. It’s the first day of astronomical spring in the Northern Hemisphere, and one of two days a year when day and night are just about equal lengths across the globe.
Because Earth is tilted on its axis, there are only two days a year when the sun shines down exactly over the equator, and the day/night line – called the terminator – runs straight from north to south.
In the Northern Hemisphere, the March equinox marks the beginning of spring – meaning that our half of Earth is slowly tilting towards the sun, giving us longer days and more sunlight, and moving us out of winter and into spring and summer.
An equinox is the product of celestial geometry, and there’s another big celestial event coming up later this year: a total solar eclipse.
A solar eclipse happens when the moon blocks our view of the sun. This can only happen at a new moon, the period about once each month when the moon’s orbit positions it between the sun and Earth — but solar eclipses don’t happen every month.
The moon’s orbit around Earth is inclined, so, from Earth’s view, on most months we see the moon passing above or below the sun. A solar eclipse happens only on those new moons where the alignment of all three bodies are in a perfectly straight line.
On Aug. 21, 2017, a total solar eclipse will be visible in the US along a narrow, 70-mile-wide path that runs from Oregon to South Carolina. Throughout the rest of North America – and even in parts of South America, Africa, Europe and Asia – the moon will partially obscure the sun.
Within the path of totality, the moon will completely cover the sun’s overwhelmingly bright face, revealing the relatively faint outer atmosphere, called the corona, for seconds or minutes, depending on location.
It’s essential to observe eye safety during an eclipse. Though it’s safe to look at the eclipse ONLY during the brief seconds of totality, you must use a proper solar filter or indirect viewing method when any part of the sun’s surface is exposed – whether during the partial phases of an eclipse, or just on a regular day.
Learn more about the August eclipse at eclipse2017.nasa.gov.
