Two equinoxes occur approximately at the same time every year. The vernal or spring equinox occurs approximately around March 20 or 21 and the autumnal equinox around Sept.. 21 or 22.
Because of the earth's tilted axis, from around March 21 to Sept. 22, the Northern Hemisphere points toward the sun. The Southern Hemisphere points toward the sun for the other half of the year. This results in opposing seasons for the Northern and Southern hemispheres.
Hour angle refers to the an observer's sidereal time during the vernal equinox at a certain location on earth. Sidereal time is a time-keeping device that astronomers use in order to identify such things as satellites and stars in the sky throughout the day. There are 23 hours, 56 minutes and four seconds in a sidereal day, which amounts to approximately the time that it takes for the earth to rotate on its own axis, with respect to the vernal equinox.
Just like the sun and the moon, stars in the night sky appear to rise in the east and set in the west. Thus astronomers often utilize hour angle to track a star's movement and location during the course of the night and to identify the same star the next night. Astronomers also take advantage of hour angle to locate and to map stars in the night sky.
Inaccuracy in measurement of date and time occurs. For example, daylight or nighttime may be a few minutes longer depending on the locations during both the March and September equinoxes. For example, Norway has 15 extra minutes of daylight during the March equinox. Seasons also are not of equal length due to the varying speed at which the earth revolves around the sun. Summer lasts 94 days from the June solstice to the September equinox but only 89 days from the December solstice to the March equinox.