The month is a unit of time and is a multiple of an SI base unit with the symbol mo.
There are 100s of definitions of a month that have been established, reviewed and adopted over the course of history but the most common around the world today are the months in the Julian and Gregorian calendars.
There are 12 months in the Julian and Gregorian year; averaging 30.4368 days in each. They are labelled January, February, March, April, May, June, July, August, September, October, November and December.
There is a famous rhyme to help remember how many days are in each month that goes “Thirty days has September, April, June and November – all the rest have thirty-one. Except February which has 28 or 29 in a leap year.”
Many cultures use the moon’s cycle (the lunar cycle) to define which month they are in.
The sidereal year is a unit of time that is a multiple of an SI unit. We have used the symbol a-sr.
From the Latin ‘sidus’ for ‘star’, sidereal is a term used to describe the orbital time of the earth or an object to complete one full cycle with respect to a fixed, distant star or constellation pattern.
The sidereal year is defined as 365.242 190 402 days of 86,400 SI seconds. This is 20 minutes and 24.5 seconds longer than a tropical year.