Kolkata: Sky enthusiasts gear up to witness the three hour long partial lunar eclipse tonight, when the Earth will move between the Sun and the Moon. Experts believe that there is no proper lunar eclipse till 2021.

The eclipse will begin around 1.31 am on July 17, the research and academic director of MP Birla Planetarium in the city, Debiprosad Duari said. The greatest partial eclipse when the Moon will look the darkest will be around 3am.

"Skywatchers should not miss this chance since there will be no proper lunar eclipse till 2021," he said.

The celestial phenomenon will be visible entirely from all parts of the country, besides parts of South America, Europe, Africa, Asia and Australia, Duari said.

The Moon will remain partially eclipsed till 4:29 am on Wednesday. So, it is a golden opportunity for sky enthusiasts in the country as the eclipse will be visible almost throughout the night, he said.

On Tuesday night, only a part of the Moon will pass the Earth's shadow. Around 3:01 am on Wednesday, 65% of the Moon's diameter will be under the shadow of the Earth, Duari said.

Elaborating on the celestial phenomenon, he said it takes place only at full Moon night, when it, the Sun and the Earth are in a perfect straight line.

As the Sun's rays fall on the Earth, its shadow falls on to a patch of space. When the Moon enters the patch of shadow there is a lunar eclipse.

The patch of shadow is actually composed of two cone-shaped parts - one nestled inside the other.

The outer shadow or penumbra is a zone where the Earth shadow is partial and blocks some, but not all of the Sun's rays. In contrast, the inner shadow or umbra is a region where the Earth blocks all direct sunlight from reaching the Moon.

When only a part of the Moon passes through the umbra, a partial lunar eclipse is seen.

If the entire Moon passes through the umbral shadow, then a total eclipse of the Moon occurs, he said.

India will witness the next lunar eclipse on May 26, 2021, when it will be a total one, he added.

When asked, he said lunar eclipses are completely safe to view with the naked eye. One does not need a telescope to watch the lunar eclipse although a good pair of binoculars will enhance the experience, Duari said.