NEW DELHI: The longest total lunar eclipse of the 21st century would occur on Friday. The eclipse will also coincide with the closest approach of Mars to Earth. Earlier this year, there was a total lunar eclipse on January 31.

For a total lunar eclipse of a long duration to occur, the moon has to pass through the central part of the Earth's shadow.

What will happen during the Lunar Eclipse 2018?

During this eclipse, the Moon will pass through the central part of the Earth's umbral shadow. Furthermore, the Moon will be at apogee -- farthest from the Earth in its orbit, on July 27 and will be moving at a slower speed in its orbit.

This slower moving full Moon will take longer time and greater distance of Earth's umbral shadow cone to travel, making it the longest duration of the total eclipse of this century.

How long the lunar eclipse will last?

The eclipse will last for 1 hour and 43 minutes, giving viewers a wonderful opportunity to experience the happening. It will be preceded and followed by partial eclipses lasting more than one hour.

The partial eclipse of the moon will start around 11.54 p.m. Indian Standard Time, with the total eclipse begining at 1 a.m. on July 28.

The greatest eclipse, when moon will look the darkest, will be at around 01:52 a.m. and the totality will continue at 02:43 a.m. after this period the moon will remain partially eclipsed till 03:49 a.m. of July 28.

How and where to watch?

Planetariums across the country have made elaborate arrangements for enthusiasts to witness the rare celestial event. The eclipse will be visible in parts of South America, much of Africa, the Middle East and Central Asia. For viewers in India, the eclipse, both partial and the total, will be visible in its entirety from all parts of the country.

Lunar eclipses are completely safe to view with the naked eye.

People in Jammu and Kashmir, and Tamil Nadu are most likely to be able to witness the event clearly, Aniket Sule, Chair of the Public Outreach and Education Committee (POEC) of ASI, told PTI.

What is blood moon?

A total lunar eclipse happens when Earth takes position in a straight line between the moon and sun, blotting out the direct sunlight that normally makes our satellite glow whitish-yellow.

The moon travels to a similar position every month, but the tilt of its orbit means it normally passes above or below the Earth's shadow - so most months we have a full moon without an eclipse.

When the three celestial bodies are perfectly lined up, however, the Earth's atmosphere scatters blue light from the sun while refracting or bending red light onto the moon, usually giving it a rosy blush.

This is what gives the phenomenon the name "blood moon".

Mars will make its closest approach to Earth in 15 years

Mars will rise around the sunset time and will set around the time of sunrise. An opposition happens when Mars is the closest to Earth in its orbit, both on the same side of the Sun.

The red planet will appear brighter and bigger than usual. Though the opposition of Mars is on Friday, it comes closest to us on July 31. This curious fact is because the orbits of all planets around the Sun are not a perfect circle, but an ellipse.

Due to this, the closest approach of Mars to us will occur four days later than its opposition. This is also one of the reasons why the distance between Mars and Earth is not the same during every opposition.

The distance between Mars and the Earth ranges between about 400-million kilometers (2.7 Astronomical Units, or AU) and 56-million km (0.38AU). The closest approaches occur during opposition.

This time, Mars comes the closest to Earth as compared to any time in the last 15 years. As a consequence, it will also appear to be the biggest and brightest in the last 15 years. At this time, Mars will be just 58 million kilometres from the Earth.

(With inputs from agencies)