A more in depth explanation from last week: If Earth had no tilt, what else would happen? •The equator would be much hotter due to the direct sunlight which would lead to a lower survival rate and little life. •The poles would receive less direct light and thus be colder making the survival rate there lower as well. •The species would have evolved differently (micro-evolution), thus different life would be on Earth. •But we would have a habitable zone between the poles and the equator, but unfortunately it would be a smaller habitable region than we have now.
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A more in depth explanation from last week:
If Earth had no tilt, what else
would happen?•The equator would be much hotter due to the direct sunlight
which would lead to a lower survival rate and little life.
•The poles would receive less direct light and thus be colder
making the survival rate there lower as well.
•The species would have evolved differently (micro-evolution),
thus different life would be on Earth.
•But we would have a habitable zone between the poles and the
equator, but unfortunately it would be a smaller habitable region
than we have now.
Solar & Sidereal Motion and
Models of the Solar System(Week 7)
Why does Sidereal Motion (Time) matter?
•Our clocks are based upon Solar time and we measure stars rising about 4
minutes earlier each day.
Sidereal Day, Sidereal Periods of Celestial Bodies to include the Sidereal
Month of the Moon
•Why does this happen?
The short version…because of Earth’s motion around the Sun.
•It is a system of timekeeping used by astronomers, useful because a star rises
and sets at the same sidereal time every day, but not at the same solar
(synodic) time which is our typical time system.
•Because local sidereal time is the right ascension (RA) of a star on the
observers meridian, it is a direct indication of whether a celestial object of
known right ascension is observable at that instant.
•What types of motion can be measured with the Sidereal system?
Sidereal Time vs. Solar (Synodic) Time
• A time-keeping system
astronomers use to keep
track of the direction to
point their telescopes to
view a given star in the
night sky.
• One sidereal day
corresponds to the time
taken for the Earth to
rotate once with respect
to a distant star.
•A time keeping system
based upon when the Sun is
highest in the sky (~12 pm).
•One solar day corresponds
to the time taken for the
Earth to rotate once with
respect to the Sun.
Prior to Tutorial completion, the Instructor will:
a) define parallel lines
b) define period
c) define high noon (in the diagram below)
d) in the diagram below illustrate a 360 degree rotation of
person/Earth with a ruler (students use toothpick) while Earth is
still orbiting the Sun & sketch the Earth/person in a later
snapshot
e) help students visualize distant stars (see
top of page) and have them draw similar
stars on their Tutorial
f) provide every student with a toothpick
Solar vs. Sidereal Day - Lecture Tutorial
(pg 11-12; 10-20 minutes)
• STOP the Tutorial just after the “Note:” on page 12, put name on it and turn it in to Instructor next Tuesday.
• Be ready to struggle a little bit, this is a discovery!
• Work with a partner!
• Read the instructions and questions carefully.
• Discuss the concepts and your answers with one another.
• Come to a consensus answer you both agree on.
• If you get stuck or are not sure of your answer, ask another group.
• If you get really stuck or don’t understand what the Lecture Tutorial is asking, ask one of us for help.
Follow up to TutorialUsing the angle that the Earth sweeps out as it goes once around the Sun
and the number of days in a year, the number of degrees per day that Earth moves in orbit about the Sun is:
A) 365 days/180 degrees = 2 days/degree
B) 365 days/180 degrees = 0.5 degrees/day
C) 360 degrees/365 days = 1 degree/day
D) 360 degrees/24 hours = 15 degrees/hour
E) none of the above
ANSWER: “C” or 1 degree/day for
Earth revolving about the Sun [Realize
that choice “D” or 15 degrees/hour is
the rotation rate of the Earth about its
axis, which is also the rate the celestial
sphere appears to rotate.]
Follow up to Tutorial
During what type of a day does the
Earth rotate through slightly
more than 360 degrees?
A) Synodic day which is 24 hrs
B) Solar day which is less than 24
hrs
C) Sidereal day which is less than
24 hrs
D) Sidereal day which is more
than 24 hrs
E) Both A) & B) above
ANSWER: “A” One solar/synodic day corresponds to the time taken for the Earth
to rotate once with respect to the Sun which is more than 360 degrees and takes 24
hours.
Follow up to Tutorial
During what type of a day does the
Earth rotate through 360
degrees?
A) Synodic day in 24 hrs
B) Solar day in less than 24 hrs
C) Sidereal day in less than 24 hrs
D) Sidereal day in 24 hrs
E) Both A) & B) above
ANSWER: “C” One sidereal day corresponds to the time taken for the Earth torotate once with respect to a distant star.
• One sidereal day lasts approximately 23 hours and 56 minutes during which time the Earthrotates 360 degrees
(~4 minutes shorter than a solar day).
• One solar (synodic) daylasts 24 hours during which time the Earthrotates more than 360 degrees.