4 Time and the Earth-Moon-Sun Cycles Flashcards
What does one year correspond to?
one year corresponds to one complete orbit of the Earth around the Sun
What is one sidereal day?
One sidereal day is the time it takes for the Earth to spin once on its axis: 23 h 56 min
What is one solar or synodic day?
The time it takes for the Earth to spin so the Sun appears in the same place in the sky: 24 h 00 min
How long is one Sidereal Month?
27.3 days
How is a Sidereal Month measured?
The time it takes for the Moon to complete one revolution of the Earth
How long is one solar (Synodic) month?
29.5 days
Why is a Synodic month longer than a Sidereal Month?
During the time it takes for the Moon to complete one orbit of the Earth, The Earth has moved through an angle of 27° in its orbit around the Sun. This is why the Moon needs an extra 2.2 days to
re-align with the Sun and Earth again
What do Sundials indicate (timing)?
Sundials indicate Apparent Solar Time (AST)
What is Apparent Solar Time?
Apparent Solar Time is the apparent timing of the Sun seen by an observer. This is in reference to the actual position of the Sun in the Sky
What do ordinary time-keeping devices indicate (timing)?
Mean Solar Time (MST)
What is Mean Solar Time?
Mean Solar Time is the time calculated by the mean time it takes for the Sun to cross the sky each day in a consistent manner.
What is Equation of Time (EOT)?
Equation of Time is the difference between mean solar time and apparent solar time, which varies with the time of year. It is used to correct Mean Solar Time to the Apparent Solar Time
What is the Sun’s ‘speed’ when the EOT is positive?
The real Sun is ‘fast’
What is the Sun’s speed when the EOT is negative?
The real Sun is ‘slow’
What can a shadow stick determine?
A shadow stick can determine at which time the Sun culminates and also the observer’s longitude.
How do you calculate Equation of Time?
Equation Of Time= Apparent Solar Time-Mean Solar Time
What is an analemma?
An analemma is a chart showing how the EOT varies with the Sun’s declination during the course of one year
What does an analemma chart look like?
An analemma is likened to a ‘snowman’ with a bigger bottom than top
How does the Earth’s orbit vary the EOT?
The Earth travels at different speeds in its orbit which causes the real Sun to move slower faster or slower in the sky on different dates
How does the tilt of the Earth vary the EOT?
When it is close to the solstices, the real Sun is travelling faster from East to West than when close to the equinoxes when a large component of its apparent motion is northwards or southwards; the Sun therefore lags behind and leaps ahead of the Mean Sun in the East-West motion.
How do you calculate the EOT?
EOT = AST - MST
