William Simpson was born on 28 October 1823 in Glasgow. Following a seven-year apprenticeship with a specialist lithographic firm, he moved to London in February 1851 and found employment with Day and Sons. In 1859 the firm commissioned Simpson to visit India and make drawings for a book illustrating well-known places associated with the 1857-58 uprising. Thus began Simpson's long association with India, and the first of his four visits to the subcontinent over the next twenty-five years.

During these journeys he made numerous rapid pencil drawings in sketchbooks, often heightened with colour washes. Many formed preparatory studies for his finished watercolours, most of which he worked up after returning to London. The plan was for Day and Son to select 250 of these finished watercolours to be lithographed as illustrations in the projected volume. While Simpson was away in India, however, Day and Son had been drifting into debt. In 1867, before it finally went into liquidation at the end of the year, he was made a company shareholder as part payment. But, as he expressed it, 'the great work on India, on which I bestowed so much time and labour, never came into existence'. Two years later, Simpson's collection of 250 watercolours was sold off as bankrupt stock: 'This was the big disaster of my life', as he ruefully remarked.

This painting depicts the Observatory in Delhi by moonlight. It was built in 1725 by the astronomer Raja Swai Jai Singh and consists of four main instruments, two having the main functionality of time measurements, and two meant principally for celestial co-ordinate measurements for the sun, moon, planets and stars.