A transit of Mercury over the disc of the Sun will take place this afternoon and be visible from Bangladesh.

A transit of Mercury across the Sun takes place when the planet Mercury comes between the Sun and the Earth, and Mercury is seen as a small black dot moving across the face of the Sun.

The transit of Mercury is relatively a rare phenomenon, occurs 13 or 14 times in a century, mostly in the months of May and November. But, the next will be visible after a long gap of 16 years in 2032, according to Nasa.

The transit will begin at around 5:00 pm and  will be seen all over the country. The first contact will take place at 17:31 and will end at 18:20.

Science organisation Anushandhitshu Chokro has set up an observation post with a solar telescope at its office to observe the transition, the organisation's President of Astronomical Division Shahjahan Mridha Benu told The Daily Star.

"The transit of Mercury is a phenomenon in which the planet will be seen as a small black dot travelling from one end of the solar disc to the other," he said.

None should see the Mercury transit with the naked eye, and staring at the sun without protection is harmful, he added.

"This historical event surely will help us to develop our Science culture and increase people's interest on Astrology."

This phenomenon is seen when mercury passes between the Sun and the Earth and it happens only when the three are lined up.

Mercury appears as a dot on the solar disc because its angular size is very small compared to that of the Sun as seen from Earth.

If someone has his own telescope, he can watch the event using a safety filter to protect the eyes from the sun. If he doesn't have a handy filter, he can use a sheet of paper to rig up a safe viewing method. He can project the image of the sun (in the form of a white disk) onto a sheet of paper, then watch the black dot of Mercury crawl across it.

The transition can be enjoyed watching online or using a telescope with a special filter.

The safest way to observe a transit of Mercury is to project the image of the Sun through a refracting telescope on to a piece of white card.