The longest lunar eclipse of the 21st Century - lasting almost 1 hour and 43 minutes - is set to take place this Friday and Saturday (July 27-28). For many in the U.S., however, the viewing will be limited to photos and online videos.

The "blood moon" eclipse, which takes its name from the red hue the moon takes on, will last four hours from start to finish.

A lunar eclipse occurs when the path of the Earth's orbit lines up directly between the sun and the moon. When this happens, the Earth blocks the light from the moon, darkening the moon with the Earth's shadow.

If the sun's rays are completely blocked, the moon takes on a ruddy red or burnt orange hue, giving it the name "Blood Moon."

The disappointing news for those in North America is the lunar eclipse won't be visible except online. To get a view of the eclipse, you would have to travel to Africa, the Middle East, southern Asia or the Indian Ocean area. According to space.com, the eclipse will be visible from eastern South America as it is ending and from Australia as it is beginning.

The time of the greatest eclipse will be 4:21 p.m. EDT on July 27. The total eclipse will last from 3:30 p.m. to 5:13 p.m. EDT.

The next total lunar eclipse visible for North America will occur on Jan. 21, 2019, with the best views for those on the West Coast. After that, there will be total solar eclipses on July 2, 2019 and Dec. 14, 2020.

How to watch the lunar eclipse

You can still watch the eclipse even if you're not going to be in one of the countries where it will be most visible. The Virtual Telescope Project will start a livestream at 2:30 p.m. ET.