Systems of Time

Solar time

Solar time is calculated based on the position of the sun in the sky. There are two types - apparent solar time and mean solar time (clocks).

Sundials are very accurate, but time changes even with a small distance. It is not reliable either as it won't work if the sun is blocked.

Clock time is standardised in a large region and works in every environment, but doesn't have the accuracy sundials have.

Sidereal time

A siderial period is the time taken for a revolution of a body about another with respect to a distant star. The earth's sidereal period is 365.25 days, so a day is added every 4 years

Synodic period

The synodic period is the period in which the phases of a celestial body as seen from another repeats itself. An example is a phase of the moon, a sunspot relative to the earth and a satellite orbiting earth.

Local meantime

Local mean time is a type of solar time using the sun's movement across the sky. It is based on the average length of a solar day, and each location has a different local time defined by its longitude. The time changes by 4 minutes every degree longitude. Due to trains, it was replaced by timezones.

Time zones

There are 24 "main" time zones, with some half-hour time zones. Each time zone is 15 degrees apart.

Coordinated Universal Time

The UTC is the starting point of all time zones at 0 degrees longitude (the prime meridian).

A day

A day is the complete rotation of a celestial object around its axis.

A year

A year is the complete orbit of a celestial body, also known as its orbital period.

Calendars

Julian

The Julian date is the number of days since the beginning of the Julain period, defined as January 1 4713BC

Heliocentric Julian date

The heliocentric Julian date is the Julian date corrected to the time at which light from a specified celestial object reaches the sun.