JOST A MON

In 1572, a massive supernova lit up the sky, outshone Venus, and stayed visible during the day. Tycho Brahe recorded it in his Stella Nova, a compendium of new stars. Contemporary astronomers (the European ones, at least) were stunned by the event as it contradicted their long-held theories of the immutability of the stellar sphere.

Although Copernicus' treatise on the heliocentric universe had already appeared thirty years earlier, he too had thought that the stars were equidistant from the Sun, fixed with respect to each other, and moving at a constant speed in an enormous solar orbit. It took a Member of Parliament from Wallingford, England, named Thomas Digges, to postulate an idea of differing stellar distances ¹.

"In 1573 Digges published Alae seu scalae mathematicae, a work on the position of the new star which is often called Tycho Brahe's supernova of 1572 since Brahe also observed the star and determined its position accurately. Digges' work includes observations of the position of the 'new star' and trigonometric theorems which could be used to determine the parallax of the star. The observations are particularly impressive making Digges one of the ablest observers of his time." ²

The appearance of a new star, while disturbing enough, was less worrisome than its gradual disappearance. Why did it begin to dim? Digges' foster-father, a natural philosopher called John Dee, suggested that the star had brightened as it approached the Earth, and dimmed as it retreated. This thesis alone put a spoke through the idea of an equidistant and unchanging shell of stars.




An illustration of the Copernican universe from Thomas Digges' book (Wikimedia Commons)

 Digges published an English version of Copernicus' treatise in 1576 (Perfit Description of the Celestiall Orbes) with some additions and elucidations of his own. His own contribution was the startling one of an infinite universe (crystallised into this one diagram), and possibly the first statement of what later came to be known as Olbers Paradox:

Clearly, if the universe was infinite and had an infinite numbers of stars, and had lasted for an infinity, every line of sight from Earth would end up in a star, and so the night sky should be completely lit up. Equally clearly, it isn't. Why?

Digges was unable to propose an entirely satisfying resolution to the problem. No shame falls to him, however, as it took nearly 350 years for the answer to emerge. With the discoveries of Edwin Hubble of the expanding universe, it became clear that the universe was not, in fact, infinite, and it hadn't existed forever, and that there were stars far out in the deep field whose light still hadn't had time to reach us.

And so we have the dark night sky.

References

1. Jim Al-Khalili, Everything and Nothing, BBC.
2. J J O'Connor and E F Robertson, Thomas Digges, MacTutor Biography.