IS1101 Nothing to Something

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IS1101 Nothing to Something
Day, Month Year

• Mr Goh Hock Leong

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Content

• Tropical Year
• Solar Day
• Sidereal Day
• Sidereal Month
• Synodic Month

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Keeping time

• Historically, the regular motion of objects in
  the sky served as the basis for timekeeping.
• The diurnal motion of the sky caused by the
  rotation of the Earth on its axis defined the
  day,
• the year was defined by the motion of the Earth
  on its orbit about the Sun, and
• the month was defined in relation to the
  revolution of the Moon about the Earth.
• Although precise modern timekeeping is done
  electronically, many of the details and the
  terminology of timekeeping remain rooted in its
  astronomical heritage.

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Keeping time

• In using the sky for timekeeping, we must define
  a reference point to determine when a cycle of
  the required motion has been completed.
• If we choose a reference point afixed to the
  celestial sphere, the corresponding time is being
  referenced to the distant stars and is termed
  sidereal time.
• If instead we choose the Sun as the reference
  point, the corresponding time is called solar
  time (or tropical time).

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Tropical Year

• Therefore astronomers defined a mean tropical
  year, that is an average over all points on the
  ecliptic
• 365.2422 SI days.
• Tropical years have been defined for specific
  points on the ecliptic in particular the vernal
  equinox year, that start and ends when the Sun is
  at the vernal equinox. Its length is about
  365.2424 days.

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Sidereal vs. Synodic Month

• The sidereal month is the time the Moon takes to
  complete one full revolution around the Earth
  with respect to the background stars.
• However, because the Earth is constantly moving
  along its orbit about the Sun, the Moon must
  travel slightly more than 360 to get from one
  new moon to the next.
• Thus, the synodic month, or lunar month, is
  longer than the sidereal month.
• A sidereal month lasts 27.322 days, while a
  synodic month lasts 29.531 days.

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Solar Day

• Time interval between two successive meridian
  crossings (noon) of the Sun. 
• Exactly 24 hrs

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Sidereal Day

• Sidereal day Time interval between two
  successive meridian crossings of any star (or
  vernal equinox).

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Solar Day vs Sidereal Day
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Solar Day vs. Sidereal Day

• The length of the solar day is variable (because
  suns orbit is not perfectly circular)
• Because of this, apparent solar days are shorter
  in Mar and Sep and longer in Jun and Dec.
• The average value of the solar day is known as a
  mean solar day.
• A mean solar day is 3 minutes 56 seconds longer
  than a sidereal day.

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Solar Day vs. Sidereal Day

• The stars rise and set on a Sidereal schedule.
• A star or constellation will be at precisely the
  same position in the sky 4 minutes earlier with
  each successive day

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Reference

• http//www.ii.metu.edu.tr/emkodtu/met204/lectures/
  section4/days.html
• http//encyclopedia.thefreedictionary.com/sidereal
  20day