Jyotish is study of Time and karma on individual. One of the important steps to understand Time is the various ways to measure it. This gives better understanding of Time's various properties or Moods.
In Jyotish, basic unit of Time is called Ahoratra (One Day and Night). Aho is Day and Ratra is Night. The various types of measure is derived from this basic units.
Various types of division is done on this Basic Unit of Ahoratra, There is division by 2 (day/night), 8(Yama), 8+8(kaala), 12 (Rashi/Bhaava), 24 (hora), 15+15 (Muhurtha), 60 (Nadi).
Let me list them out, in regular measure of hours, for your easy reference in your future studies,
|Unit||Normal Duration||Brief Description|
|Ahoraatra||24 Hours||Day and Night|
|Dina Maana||12 Hours||Day(Aho) Duration|
|Raatri Maana||12 Hours||Night(Raatri) Duration|
|Raashi/Bhaava||2 Hours||12th part of Ahoratra|
|Hora||1 Hour||12th Part of Dina or Raatri Maana, Hora from Aho+Ratra|
|Yama||3 Hours||8th part of Ahoraatra|
|Kaalam||1.5 Hours||8th part of Dina Or Raatri Maana|
|Muhurtha||48 Minutes||15th Part of Dina or Raatri Maana|
|Naadika||24 Minutes||60th Part of Ahoraatra|
|Ghatika||Naadika||Also called Ghati or Naadi|
|Praana pada||6 Minutes||Pada means quarter, It's Quarter of a Naadi. It's 20th of a Bhaava.|
Significance of each for of Division,
Divide by 2: Jyotish shastra also called Hora Shastra basic division is by 2. Where each creations Purusha and Prakriti influence is noted.
Divide by 12: Twelve is number associated with Sun. Divide by 12 is called Arkamsha or Suryamsha. Sun is supposed to have 12 forms.
Divide by 8: This is derived from 8 spokes of Kaala chakra, The divide by eight shows Kaala's blocking nature. The 8 signifies Praana or longevity, the 8th house in Jyotish denotes it. It's Time measured in Praanas or breaths in human lives.
Kaalam unit of times is derived from this. Units indentified thus are like Raahu Kaalam, Gullika Kaalam etc.
Divide by 15: The jyotish term for divide by 15 is called Tithi amsha. Tithi means 15 to denote the 15 faces of Moon.
Further sub-division of the above units can also be made. Such division's nomenclature is by adding prefixes to above notations. For example, "Vi" is prefix is to indicate further division of the unit by 60, Like Vi+Naadika or Vi+Ghaatika, Vinaadi/Vighatika is 60th part of 24 Minutes hence is 24 Seconds.
The Above measures of time is used to derived various divisional chart in Jyotish and the influence on the person. The time period which the person is "sticking to" Lagna is called Rashi Lagna, Bhaava Lagna, Hora Lagna, Ghatika Lagna, Vighatika Lagna etc.
Above given are some of the basic demarcations of time. Detailed study can be made from books on Panchanga.