The first total lunar eclipse of the year happens this Sunday and the United States is in prime position for optimal viewing.

We see a full moon when the path of the Moon about the Earth lines up with the path of the Earth about the Sun, such that the illuminated face of the Moon is visible from Earth. This full moon alignment happens approximately every 29 days and is technically known as syzygy of the Sun-Earth-Moon-system. When the Moon passes directly behind the Earth into its shadow, we see an eclipse. However, when this alignment is exact, such that the Suns light is completely blocked by the Earth’s shadow, we see a total eclipse. During a total eclipse, the only light that we see reflected from the Moon is light that has passed through the Earth’s atmosphere. In an effect known as Rayleigh scattering – where light is scattering with no effect on its wavelength – the Sun’s rays interact with the Earth’s atmosphere scattering the light in different directions. The more energetic blue rays, with a shorter wavelength, are the most affected, whereas lower energy rays, with longer wavelengths, pass straight through the Earth’s atmosphere where they are refracted around the Earth to the Moon. It is these refracted rays that give the Moon a reddish-orange colour during an eclipse – which is sometimes referred to as a blood moon.

The eclipse this Sunday 20th January is a total lunar eclipse – a blood moon. As well, due to its current proximity to Earth the moon will be a supermoon and thus even more spectacular than normal. The moon follows an elliptical orbit about the Earth, varying between 221,643 miles (356,700 km) away at perigee and 252,463 miles (406,300 km) at apogee. As the moon is currently at perigee it will appear slightly larger and brighter than normal and is thus referred to as a supermoon.

The total eclipse this Sunday 20th January will be visible from the entire Americas starting at 6:36 pm PST, reaching totality at 8:41 pm PST which will last for 62 minutes peaking at 9:12 pm PST, and finally with the full event finishing by 11:48 pm PST. For up-to-date viewing information and times see here.

By: Amira Val Baker, RSF research scientist