Amateur astronomers will search the night sky for nebulae and star clusters
Stargazers in the north are to attempt to count as many Messier Objects - also known as faint fuzzies - as they can in 11-and-a-half hours.
The list, first compiled and named after 18th Century French astronomer Charles Messier, includes nebulae and galaxies.
Numbering more than 100, they are among the hardest objects to spot.
Highlands Astronomical Society and Moray group Sigma will attempt their Messier Half Marathon on 27 March.
From 1830 GMT until 0600 the following morning, members will try to find as many on the catalogue as they can.
Messier put his list - which also include star clusters - together from 1758 to 1782.