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(Of course, if the planet rotates in the opposite direction that its revolution around the primary, the solar day will be shorter than the sidereal day, with the same amount. Most planets don't do this -- they rotate around the axis and revolve around the primary in the same direction.)

As the planet revolves around its primary there are four important points on the orbit;

(Of course, if the planet rotates in the opposite direction of its revolution around the primary, the solar day will be shorter than the sidereal day, with the same amount. Most planets don't do this -- they rotate around the axis and revolve around the primary in the same direction.)

As the planet revolves around its primary there are four important points on the orbit:

• The simplest way is the time required for the planet to complete a full rotation around its axis; this is called a sidereal day, because it is the time between two culminations of any given star. ("Sidera" means stars in Latin.)

  You choose the length of the sidereal day. It can shorter than an Earth day, it can be longer. There is no relationship between the length of the year and the length of the sidereal day.

• The more complicated, but more useful way is to reckon the time between two culminations of the planet's sun, that the time from one noon to the next; this is called a solar day, and it is a bit longer than a sidereal day, and can be calculated as the duration of one sidereal day plus a fraction of the sidereal day equal to the ratio between the sideral day and the year.

• For Earth, a sidereal day is 23 hours 56 minutes, and an average solar day is 24 hours. (The current definition of the second was chosen very carefully, so that the mean solar day computed for 1 January 1900 is almost exactly 24 hours.)

• The simplest way is the time required for the planet to complete a full rotation around its axis; this is called a sidereal day, because it is the time between two culminations of any given star. ("Sidera" means stars in Latin.)

  You choose the length of the sidereal day. It can shorter than an Earth day, it can be longer. There is no relationship between the length of the year and the length of the sidereal day.

• The more complicated, but more useful way is to reckon the time between two culminations of the planet's sun, that is, the time from one noon to the next; this is called a solar day, and it is a bit longer than a sidereal day, and can be calculated as the duration of one sidereal day plus a fraction of the sidereal day equal to the ratio between the sideral day and the year.

• For Earth, a sidereal day is 23 hours 56 minutes, and an average solar day is 24 hours. (The current definition of the second was chosen very carefully, so that the mean solar day computed for 1 January 1900 is almost exactly 24 hours.)

A day can be reckoned in two ways. The simplest way is the time required for the planet to complete a full rotation around its axis; this is called a sidereal day, because it is the time between two culminations of any given star. ("Sidera" means stars in Latin.) The more complicated, but more useful way is to reckon the time between two culminations of the planet's sun, that the time from one noon to the next; this is called a solar day, and it is a bit longer than a sidereal day, and can be calculated as the duration of one sidereal day plus a fraction of the sidereal day equal to the ratio between the sideral day and the year.

• For Earth, a sidereal day is 23 hours 56 minutes, and an average solar day is 24 hours. (The current definition of the second was chosen very carefully, so that the mean solar day computed for 1 January 1900 is almost exactly 24 hours.)

A day can be reckoned in two ways.

• The simplest way is the time required for the planet to complete a full rotation around its axis; this is called a sidereal day, because it is the time between two culminations of any given star. ("Sidera" means stars in Latin.)

  You choose the length of the sidereal day. It can shorter than an Earth day, it can be longer. There is no relationship between the length of the year and the length of the sidereal day.

• The more complicated, but more useful way is to reckon the time between two culminations of the planet's sun, that the time from one noon to the next; this is called a solar day, and it is a bit longer than a sidereal day, and can be calculated as the duration of one sidereal day plus a fraction of the sidereal day equal to the ratio between the sideral day and the year.

• For Earth, a sidereal day is 23 hours 56 minutes, and an average solar day is 24 hours. (The current definition of the second was chosen very carefully, so that the mean solar day computed for 1 January 1900 is almost exactly 24 hours.)