A4 - Time and Earth-Moon-Sun cycles Flashcards
Why is a synodic day different to a sidereal day?
-sidereal is how long it takes for one full rotation of the Earth
-synodic is how long it takes for the Sun to cross the same meridian on consecutive days
-synodic is slightly longer (24hrs) due to the extra angle to get the Sun back in the same position in the sky, as it moves along the ecliptic through the orbital motion of the Earth
A synodic day is the same as a solar day
How is the Apparent Solar Time (AST) determined? Why is this not always the best way to measure the time?
-sundial (pointed at the NCP/SCP) creates a shadow at the corresponding time
-might be nighttime/cloudy/sky is obstructed
-EOT means it is usually a little bit ahead/behind
Give reasons for EOT in a question
Give some ways of ensuring that a sundial will measure AST accurately:
-gnomon must point to the NCP
-sundial must be level
What is Mean Solar Time (MST)?
the time found when using the theoretical “mean Sun” that moves across the sky at a constant rate
ie local mean time
What does the Equation of Time (EOT) indicate?
-it tells you how far ahead the AST (from the sundial) is compared to the MST
MST = AST - EOT
eg if the EOT is -12, then the sundial is -12 minutes ahead, so 12 minutes behind
Give 2 reasons for the EOT:
eccentricity of Earth’s elliptical orbit - changing distance between Earth and Sun means Earth’s orbital speed changes, so Sun’s apparent speed across the sky changes
obliquity/tilt of ecliptic - tilt between rotational axis and orbital plane changes the altitude of Sun throughout the year, so its angular speed across the sky changes
The eccentricity means Earth’s orbital speed changes, and this also changes the length of a solar day - the faster the Earth moves in its orbit, the more it has to rotate to get the Sun back on the meridian.
Describe an observational procedure that could be done to investigate how the EOT changes through the year:
-EOT is AST-MST
-find AST from sundial pointed at the NCP
-find MST from a clock and calculate EOT
-don’t look at the Sun
-repeat at the same location and time every day over the course of a year
How does AST depend on longitude?
-the further East you are, the earlier you see the Sun rise
-therefore the AST in the East is always ahead of Universal Time (ie GMT)
Remember that Japan is the Land of the Rising Sun
How do you determine your longitude on land?
-take measurements of a shadow stick around midday to find when it is shortest, and calculate the corresponding MST to your apparent solar noon using the EOT
-record the GMT at that time using a clock (ie the MST on the Prime Meridian)
-get difference in time between your MST and GMT
-know that 4 mins = 1°
-calculate angle corresponding to the difference in time, ie your longitude
-if your MST is ahead of GMT, you are East of the Prime Meridian
-if your MST is behind GMT, you are West of the Prime Meridian
Find when the shadow is shortest using a graph
Describe the structure of a sundial:
gnomon points to NCP/SCP
What is GMT/UT/UTC defined as?
the MST at any point on the Prime Meridian
What is a sidereal month?
the time it takes for the Moon to orbit the Earth once (27.3 days)
How is a synodic/solar month different to a sidereal month? How long is it?
-synodic month is the time difference between 2 consecutive New Moons (29.5 days, a bit longer than sidereal)
-it is longer because the Earth-Moon system orbits the Sun so the apparent position of the Sun changes, and an extra 27° of Moon orbit is needed to get back to the place for a new Moon
How does the lunar distance method of determining longitude work?
-find angular separation between Moon and a star on the ecliptic
-find angle in an almanac (fancy book) and a corresponding time would be found, giving the sailor GMT
-find the altitude of the same star, and use almanac to get MST
-time difference between the MST and GMT can be used to find the longitude (4 minutes of difference = 1°)
Who invented the marine chronometer?
John Harrison
What did John Harrison do to make keeping time at sea easier? What else was it used for?
-made a marine chronometer that kept the time accurately (used oscillating spring rather than a pendulum)
-used in the horological method of finding longitude
Horology is the study of the measurement of time
