determine whether their spatial extension is "small" or "large", in the sense that notable fluctuations to an object as vast as our Milky Way cannot be seen over a small time period, such as a few years, whereas such substantial changes are possible if the size of the object does not exceed a diameter of a few light-years. Lampland's comments were confirmed some weeks later by John Charles Duncan, an astronomer at the Mount Wilson Observatory. He benefited from photographic material obtained with equipment and emulsions that had not changed since 1909; as a result the comparison with older snapshots was easy and emphasized a general expansion of the cloud. The points were moving away from the centre, and did so faster as they got further from it. Also in 1921, Knut Lundmark compiled the data for the "guest stars" mentioned in the Chinese chronicles known in the West. He based this on older works, having analysed various sources such as the Wenxian Tongkao, studied for the first time from an astronomical perspective by Jean-Baptiste Biot in the middle of the 19th century. Lundmark gives a list of 60 suspected novae, then the generic term for a stellar explosion, in fact covering what is now understood as two distinct phenomena, novae and supernovae. The nova of 1054, already mentioned by the Biots in 1843, is part of the list. It stipulates the location of this guest star in a note at the bottom of the page as being "close to NGC 1952", one of the names for the Crab Nebula, but it does not seem to create an explicit link between them. In 1928, Edwin Hubble was the first to note that the changing aspect of the Crab Nebula, which was growing bigger in size, suggests that it is the remains of a stellar explosion. He realised that the apparent speed of change in its size signifies that the explosion which it comes from occurred only nine centuries ago, which puts the date of the explosion in the period covered by Lundmark's compilation. He also noted that the only possible nova in the region of the Taurus constellation (where the cloud is located) is that of 1054, whose age is estimated to correspond to an explosion dating from the start of the second millennium. Hubble therefore deduced, correctly, that this cloud was the remains of the explosion which was observed