Topic 4 - Time and the Earth-Moon-Sun cycles

• How long is a solar day?

  24 hours on average
• how long is a sidereal day?

  23 hours 56 minutes
• what is one sidereal day?


  the time taken for earth to spin once on its axs
• what is a synodic day?

  the time taken for a planet to rotate once in relation to the star it is orbiting ( known as solar day)
• what causes the difference between solar day and sidereal day?

  due to the rotation of the earth about the sun
• with respect to the stars the earth rotates through 360 degrees in 23h 56m, but during this time, the earth has moved around the sun by about 1 degree
• in one sidereal day the stars will appear to be in the same position in the sky, whereas in one solar day the sun appears in the same position in the sky
• how many more minutes will the earth need to rotate to align with the sun once again (after it has rotated on its axis once)?

  4 minutes
• the length of the solar day can change throughout the year by up to..?

  16 minutes
• what causes the variation in the length of the solar day?


  earths elliptical orbit

  earths axial tilt

  earth moves at different speeds in different seasons
• during what months are apparent solar days shorter?

  march or September
• during what months are the apparent solar days longer?

  June or December
• how can apparent solar time be measured?

  sundial or shadow stick
• what is apparent solar time?

  AST is based on the current position of the sun as it appears to an observer in a particular location on earth - based on the apparent motion of the actual sun
• the interval between two successive returns of the sun to the local meridian is known as...?

  apparent solar day
• Mean solar time is used as the average time the sun takes between successive crossings of the meridian and lasts?

  24 hours
• how is mean solar time measured?

  by most clocks and watches
• what is the aphelion?

  the point in an orbit furthest away from the sun
• at what point in its orbit does the earth move slower?

  aphelion
• at what point in its orbit does the earth move fastest?

  perihelion
• the length of a solar day changes throughout the year by up to 16 minutes, these days are called..?

  apparent solar days
• why should AST be thought of as local apparent solar time?

  because AST it is dependent on where you are on the earth and the position of the sun there
• MST creates a ‘fictional’ Sun, or mean Sun that moves along the celestial equator at a uniform speed.
• what is local mean time?


  the mean solar time for a specific location on earth
• which locations have the same LMT?

  those that have the same longitude
• what is the equation of time?

  the difference between the mean solar time (MST) and apparent solar time (AST)
• how do we calculate equation of time (EOT)?

  equation of time= apparent solar time - mean solar time
• what is a shadow stick?

  a thin cylindrical pole with a circular cross section
• where is the shadow stick placed?

  vertically and stably onto a piece of light coloured card onto which the stick will cast its shadow
• the shortest shadow will show?

  local noon
• what is local noon?

  the time at which the sun culminates
• what two features make up a sundial?

  dial

  gnomon
• what is marked on the dial?

  hour lines to indicate time of day
• where does the gnomon point?

  north celestial point
• the angle between the slanting edge of the gnomon and the dial shows..?

  the observers latitude
• the time taken for the moon to make one complete revolution of the earth (its orbital period) is known as..?

  one sidereal month
• a synodic (solar) month is the time interval between?


  one full moon and the next - the duration of the lunar phase cycle,
• during the winter solstice the sun will lie directly above the..?

  Tropic of Capricorn
• when will the sun lie directly above the tropic of cancer?

  summer solstice
• when does the sun lie directly above the equator?

  equinoxes