The Science of a Solar Eclipse

The Moon is a cold, rocky body 238,855 miles from the Earth. It has no light of its own but reflects sunlight off its surface. Orbiting the Earth every 29.5 days, the Moon cycles through a series of phases as it reflects different amounts of sunlight, depending on the relative angle with the Sun. During certain phases of the Moon, during either a New or Full Moon, a phenomenon known as an eclipse can occur. 

Solar eclipses occur during a New Moon, when the Moon travels between the Earth and the Sun, casting a shadow on the Earth’s surface. There are three types of solar eclipses: total, partial and annular. Lunar eclipses occur when the Earth travels between the Moon and the Sun. The Earth therefore casts a shadow on the Full Moon’s surface. Eclipses are not a monthly feature, however, because the Moon’s orbit is tilted approximately 5 degrees relative to Earth’s orbit around the Sun, also known as the ecliptic. These orbits must intersect in order for the three bodies to align, resulting in an eclipse.

A composite image showing the progression of a total solar eclipse in Madras, Oregon on Monday, August 21, 2017. Photo Credit: (NASA/Aubrey Gemignani)

During a total solar eclipse, the Moon fully obscures the Sun. Totality is a narrow path across Earth’s surface where all direct sunlight is blocked, essentially turning day into night. It is only during totality that the Sun is safe to view with the naked, unprotected eye. For a few precious minutes the streamers of the corona, the outer layer of the Sun’s atmosphere, visibly dance around a silhouetted Moon. Total eclipses are important for research about the corona since this is the only time when this faint characteristic of the Sun is visible.

One critical reason that total solar eclipses are possible is due to the fact that the Sun is approximately 400 times larger than the Moon, yet also 400 times farther away, making both objects relatively the same size in the sky to observers on Earth.

During a partial solar eclipse, the Moon only partly conceals the Sun. It is a spectacular event, nonetheless, as daylight suddenly dips and temperatures may drop several degrees. A partial eclipse is dangerous to observe without eye protection as part of the Sun’s surface is still exposed. 

When the Moon is at apogee, the furthest point from the Earth in its orbit, the Moon becomes slightly smaller in the sky. Therefore the Sun is relatively larger than the Moon. If a solar eclipse takes place when the Moon is at apogee, this is known as an annular eclipse. Totality does not occur and a ring of light shines around the Moon. However, the corona is not visible during an annular eclipse because part of the Sun’s surface remains exposed, dimming the light of the corona. It remains unsafe to look at the Sun without eye protection during an annular eclipse. 

On Tuesday July 2, 2019, a total solar eclipse will produce a 124 mile-wide path of totality stretching from coast to coast across Chile and Argentina. Watch it live at NASA and Exploratorium’s eclipse portal and see this amazing phenomenon first-hand.

–Cody Norberg