The term watch is used in several ways in the Royal Naval Service. First, it is used to signify a portion of the crew which is on duty at a given time. There are seven watches during the day, five of which are four hours long, and two of which are two hours long. The two hour long "dog watches" serve to create an uneven number of watches, so that the same officers do not serve on the same watch every day. Since "day" and "night" have only arbitrary meaning on a starship, the ship's time is set to be equivalent to Enaj Mean Time. The watches are divided as follows: