Is there any evidence that Delphinus was used to keep time?

For all the speculation that Delphinus was used to time rituals, there is a still a major problem. Namely that they would appear to be a daft bunch of stars to choose. They glimmer at around magnitude 3.5 to 4, which is faint. There are hundreds of brighter stars, so can we be sure that these were observed to mark the year and if so why?

The easiest question to answer is “Was Delphinus used to mark time in the ancient world”, because the answer is yes. At least the answer is known to be yes at least as early as the fifth century BC. We know this because around this time two Greek astronomer working in Athens created a parapegma.

A parapegma is a stone pillar. It has holes carved into it to hold a wooden peg, and next to each hole a description of the day. As the days pass, so you would move the peg from hole to hole, making it a public calendar. By the sides of these holes would be other notes. These could be comments on the weather, like “Today the wind comes from the north”. There were also astronomical observations like “Today the Bird [Cygnus] is half risen.” Both the Parapegma of Euctemon and the Parapegma of Democritus both refer to Delphinus on more than one occasion.

Why is a much more difficult question. A couple of reasons come to mind and both or neither may be right.

One is that Delphinus rises at a convenient time. Around 750 BC it was rising within a couple of days of the winter solstice. Spotting the solstice by looking at sunrise alone is quite difficult. The solstice is, by definition, the time of year when the sun rises day after day in the same place. With modern astronomy we can pin the date of the solstice to one day, but for the ancients it may have been more a period of a several days. With the rising Delphinus coinciding with the winter solstice it may be been in the right place at the right time to be useful. However, bad weather could obscure this rising, so the heliacal rising wouldn’t be a good to rely on by itself.

The other reason Delphinus might be useful is that by a coincidence the date of its heliacal setting, the last time it is visible in the evening sky, more or less matches the date of the heliacal rising (depending on atmospheric conditions). This effectively means that the one constellation offers two events to observe around the same day. If it is raining in the morning, you can still look for the constellation setting in the west in the evening if the weather has improved.

This might seem true for all stars, but this is not the case. Stars in the southern celestial hemisphere are totally obscured by the sun for a few days during the year. Stars in the north can be seen on any night of the year, if you wait up long enough. Delphinus is just in the right position that the setting and rising dates match, due to its location on the celestial sphere, the position of the sun at that time of year, and their brightness being just right.

Usually when we look for reasons for constellations to exist, we look for bright stars, or things that make obvious shapes. It would seem that with a lifetime of experience observing the stars other patterns were observed and the ancients were keenly aware of the place of the stars in time, within the turning of the seasons.