Time and The Earth Moon Sun Cycles Flashcards
(15 cards)
What is the difference between a solar(synodic) and sidereal day
1 sidereal day is the time taken for 1 full rotation of Earth on it’s axis (23h 56min)
1 Solar day is the time taken for the Sun to return to the same point in the sky (24hr 00min)
Apparent Solar Time (AST) is
The local time indicated by a sundial
Why must a mean sun be used
Because the sun doesn’t move evenly like our clocks, so a mean is taken as if the sun movened at a constant speed,
Equation of Time =
AST - MST.
This value varies annually because of the Earths elliptical orbit and axial tilt
How to determine local noon
Measure the length of a shadow stick overtime
The lowest point on a length - time curve is the local noon
How do sundials work
Sundials use a triangular gnomon with an edge pointing to the North Celestial Pole. The sun shadow falls on a horizontal plate which indicates the local AST.
What is the lunar phase cycle
New - Waxing Crescent- First Quarter - Waxing Gibbous - Full - Waning Gibbous - Thurd Quarter- Waning Crescent - New
What is the difference between solar(synodic) and sidereal months?
One sidereal month is 27.3 days, the time taken for the moon to complete 1orbit of Earth
One solar month is 29.5 days, the time take to complete a lunar cycle
What causes the annual variation in times of sunrise and sunset?
The Earths axial tilt, sometimes it is tilted away from or towards the sun, causing different day lengths.
What is the astronomical significance of equinoxes and solstices
Equinoxes mark the point where the celestial equator crosses the ecliptic, and day and night are almost equal
Solstices mark the Sun’s highest or lowest declination
How does the suns apparent motion vary throughout the year
Between the summer and winter solstices, the sun is getting lower in the sky, but from winter to summer it is getting higher
Describe longitudes and time zones
At different longitudes, there is a different local time. 1 degree of rotation is equivalent to roughly 4 minutes.
Therefore we use time zones, with each 1 hour time zone occurring at roughly 15 degrees of longitude 15 degree intervals, centred around the Prime Meridian in Greenwich.
We use time zones to ensure that clock time corresponds to the position of the sun for different locations on Earth
How to determine longitude from shadow stick data and the equation of time
- Find local noon using a shadow stick
- Find solar noon in Greenwich using 12:00 + EOT
3 Compare your local solar noon to Greenwich solar noon and convert time to degrees
What is the lunar distance method
It involves measuring the angle between the moon and a known star or planet, and comparing this to observed in Greenwich,to find longitude
Describe the horological method (Harrison’s Marine Chronometer)
- Observe local noon
- Use a chronometer to check the GMT time at the moment
- Compare the two
