Astronomy Data Glossary

 

 

• Julian Date - The Julian date (JD) is the interval of time in days and fractions of a day since January 1, 4713 BC Greenwich noon
  • See this web page for more info.
    
    
• UTC - Coordinated Universal Time (abbreviated UTC)[1] is the time standard by which the world regulates clocks and time. Computer servers, online services and other entities that rely on having a universally accepted time use "Coordinated Universal Time" for that purpose.
  • This site has more info on UTC
    
    
• Age of Moon - This is the time that has elapsed since the last new moon.
  
  
• Moon Phase - The phase of the moon is the appearance of the illuminated portion of the Moon as seen by an observer, usually on Earth. The lunar phases vary cyclically as the Moon orbits the Earth, according to the changing relative positions of the Earth, Moon, and Sun. One half of the lunar surface is always illuminated by the Sun (except during lunar eclipses), and hence is bright, but the portion of the illuminated hemisphere that is visible to an observer can vary from 100% (full moon) to 0% (new moon).

 

 

 

 

 

 

 

• Moon Distance - In astronomy, a lunar distance (LD) is a measurement of the distance from the Earth to the Moon. The average distance from Earth to the Moon is 384,403 kilometers (238,857 miles). The actual distance varies over the course of the orbit of the moon.
  
  
• Moon Size (in degrees)- The angular diameter of the Moon, when seen from Earth is approximately 0.49 degrees ( 29 arcminutes or 1769 arcseconds)
  
  
• Sun Distance (in AU) - This is the distance from the Earth to the Sun, roughly equal to 1 Astronomical Unit (AU) (92,955,807.27 miles).
  
  
• Sun Size (in degrees)- The angular diameter of the Sun, when seen from Earth is approximately 0.53 degrees ( 32 arcminute or 1920 arcseconds)
  
  
• Lunation - Brown's Lunation Number, which defines lunation 1 as beginning at the first new moon of 1923, the year when Brown's lunar theory was introduced in the major national astronomical almanacs. Lunation 1 occurred at approximately 02:41 UTC, January 17, 1923.
  • More information is available here
    
    
• Pesach - Hebrew for Passover.
  
  
• Sun Transit - The sun transit time, also referred to as sun-meridian transit time, is the time at which the sun passes over the observer's meridian line. Most often this time is used as local noon time and therefore will differ from longitude to longitude. For example, on April 29 the sun transit time in Boston, Massachusetts is 12:42 p.m., whereas the sun transit time for New York City, New York is 12:53 p.m. This is due to longitudinal distance between the two cities.
  
  
• Perihel - The point in time where the Earth comes nearest to the Sun.
  
  
• Aphel - The point in time where the Earth is farthest away from the Sun.
  
  
• Perigee - The point in time where the Moon is nearest to the Earth.
  
  
• Apogee - The point in time where the Moon is farthest from the Earth.
  

 

Animation of the Moon as it cycles through its phases, as seen from the Northern Hemisphere. The apparent wobbling of the Moon is known as libration.