Hydra is the water snake (or water serpent), which in Greek mythology lived in Lake Lerna near Argos in southern Greece, and was killed by Hercules as one of his twelve labours. Crater is the cup of the Greek god Apollo, who was attended by a crow, namely Corvus. Sextans is a modern constellation introduced by Johannes Hevelius of Danzig to immortalise his sextant which was destroyed when his house-observatory was burnt down in 1679.

Despite (or perhaps because of) being the largest constellation in the sky, Hydra is virtually impossible to make out. It is low in our skies at the best of times, it appears as the evenings start to get lighter and the clocks go forward, and it only has one second-magnitude star and five third-magnitude stars, all strung out on a long chain. With binoculars you should be able to make out the small pentagon which is the head of the water snake below Cancer. Its brightest star Alphard means 'the solitary one' in Arabic, and that is apt. It is a yellow giant, and the third largest star in the sky after Betelgeuse and R Doradus.

Crater does indeed look like a chalice, but the stars in the base are fourth magnitude, and the ones in the bowl are all fifth magnitude. In a dark sky it is a lovely asterism, but no longer so in Havering (unlike when I was young and the streetlights went out at 1am). Corvus is a quadrilateral of third magnitude stars and can be made out, resembling a resting crow. Sextans is just a line between Alpha and Beta Sextantis. 

There are two relatively easily observed Messier objects in Hydra. The star cluster Messier 48 will be considered under Monoceros (it lies on the boundary between Monoceros and Hydra), as it is best observed when Monoceros is easily seen. Messier 68 [1] is a rather dim globular cluster which also suffers from being far south, with a declination of almost 27 degrees south. The barred spiral galaxy Messier 83 is even worse, being 30 degrees south and not possible to observe in Havering given the baleful effect of extinction. The Ghost of Jupiter [2] planetary nebula is further north (declination of 19 degrees south), and can be observed with a small telescope. It was discovered by William Herschel in 1785. Appearing as a blue blob in a small telescope, it is one of these early planetary nebulae which gave rise to the confusing term.

There are no easily observed deep-sky objects in Corvus, Crater or Sextans. The famous Antennae Galaxies (Caldwell 60/61; NGC 4038/NGC 4039; Arp 244) lie between Corvus and Crater, but they are not easily seen in small telescopes; nor is the Spindle Galaxy (Caldwell 53, NGC 3115) in Sextans, above Lambda Hydrae.

Hydra has two interesting variable stars. V Hydrae [3] is a dying red giant which every 8.5 years unleashes balls of fire. It is also one of the reddest stars in the sky, with a B-V value of 5.4, and is called a carbon star. R Hydrae [4] is another dying red giant which is a Mira variable, varying from an easily spotted magnitude 4.5 to a practically invisible 10; its period is 385 days.

There are several double stars of note in Hydra. Tau1 Hydrae [5] is a very wide double with a yellow primary (mag. 4.6) and a dim secondary (mag. 7.3) which has been described as blue or plum-coloured. F Hydrae [6] is similar to Tau1 Hydrae, being very wide with a mag. 4.7 yellow primary and a dim (mag. 8.2) blue secondary. 27 Hydrae [7] is even wider, with a separation of 229 arcseconds, with an orange primary (mag. 4.9) and a white secondary (mag. 7.0). Another very wide double is Chi Hydrae [8] (also known as 9 Crateris), which is a yellow (mag. 5.0) and blue (mag. 5.7) pair, separated by 507 arcseconds, and which are an optical double. By contrast, N Hydrae [9] (also known as 17 Crateris) is a pair of equal (mag. 5.6 and 5.7) yellow-white stars with a modest separation of 9.6 arcseconds.

Gamma Crateris [10] is a tight double (4.4 arcseconds) with a white primary (mag. 4.1) and a dim secondary (mag. 7.9) which is grey or red. Algorab (Delta Corvi) is one of the Astronomical League’s 100 double stars, but the secondary is dim (mag. 8.5). However, it has a good separation (24.2 arcseconds) and a fairly bright primary (mag. 3.0); the colours are yellow and purple. Our last double is in Virgo, but it is much closer to Hydra. 54 Virginis [11] is a pair of white stars with similar brightness (mag. 6.8 and 7.2) and a separation of 5.4 arcseconds.