Written by By Simeon Gholam, CNN
With a duration of almost two hours and 43 minutes, the lunar eclipse this weekend will be the longest one in nearly 600 years.
The total lunar eclipse — called a blood moon or supermoon — will occur on May 21 and usher in the summer solstice, which marks the beginning of the longest day of the year and will allow much longer daylight hours.
The next supermoon will occur on August 21, which will be especially close, allowing for the alignment of the full moon and the sun, according to the moon team at NASA.
Why is it called a blood moon?
The term “blood moon” was first used in 1925, according to Leland MacDougall, lunar eclipse historian and founder of The Lunar Eclipse Observer , a popular lunar eclipse website.
This is because the moon looks reddish during an eclipse, and this color was first tied to the radioactive decay of uranium from the volcanic remnants of Tunguska, Russia, in 1908, when some 11 million tons of ash were ejected from an eruption, according to the moon team . This color was later used by ancient cultures as a sign of death and a sign of the new moon being far away.
To make a total lunar eclipse, the moon passes through the Earth’s shadow. Using a telescope or binoculars, you can look for the crescent moon, which will be small at the beginning of the eclipse, slowly getting larger and larger as it moves through the Earth’s shadow.
The red hue comes from two things: radioactive potassium diphosphate in the moon’s atmosphere, as the moon passes through Earth’s shadow, and sunlight striking the red Earth, says Anthony Gonzales, a moon historian and senior astronomer at the Hayden Planetarium in New York.
The ‘annulus’ will take on more brightness as the eclipse progresses. (Photo Credit: Bobby Curtis/Cultura Creative/Getty Images)
The outer part of the Earth’s shadow, called the penumbra, will start to darken the moon about 20 minutes after moonset and continue to darken it until a few minutes before moonset at 5:35 a.m. ET. Then, during the first hour of totality, the red color fades away as the moon is captured by the Earth’s shadow, but then the colors begin to change again in the second hour of totality, which is when the total eclipse occurs.
The last total lunar eclipse that lasted two hours and 13 minutes was in 1833, and the longest eclipse with a duration of two hours and 22 minutes was in 1833 and 1847, according to NASA.
When to watch?
The weather should be clear across the US and Canada on Saturday morning. During totality, it won’t be dark outside like most evenings, but you will need to use special viewing glasses or binoculars.
Many cities are hosting watch parties for the blood moon, including Minneapolis, Kansas City, Los Angeles, and New York. In Washington, a ” total eclipse park” will be set up, similar to the Lakeview Park Supermoon watching party that we reported on in January, which was a surprising, peaceful, and stunning spectacle.