Download Lecture04

ASTR 1101-001
Spring 2008
Joel E. Tohline, Alumni Professor
247 Nicholson Hall
[Slides from Lecture04]
Assignment: “Construct” Scale Model of
the Solar System
• Sun is a basketball.
• Place basketball in front of Mike the Tiger’s habitat.
• Walk to Earth’s distance, turn around and take a picture
of the basketball (sun).
• Walk to Jupiter’s distance, take picture of sun.
• Walk to Neptune’s distance, take picture of sun.
• Assemble all images, along with explanations, into a
PDF document.
• How far away is our nearest neighbor basketball?
Due via e-mail ([email protected]): By 11:30 am, 25 January (Friday)
You may work in a group containing no more than 5 individuals from this class.
What about the Dime?
NOTE: A dime held 1 meter from your eye subtends an angle of 1°.
Information on Planets
[Drawn principally from Appendices 1, 2 & 3]
Planet
Earth
Mars
Jupiter
Mercury
Venus
Uranus
Saturn
Neptune
Rotation Period
(solar days)
1.00
1.026
0.414
58.646
243 (R)
0.718 (R)
Orbital (sidereal)
Period
(solar days)
Inclination of
equator to orbit
(degrees)
“Moon’s” orbital
period
(solar days)
365.25
687.0
23°
25°
27.32
4331.86
87.97
224.70
30,717.5
3°
½°
177°
98°
Two satellites:
0.319 & 1.263
Thirty-nine satellites!
No satellites 
No satellites 
Twenty-seven
satellites!
More Details on …
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Earth’s rotation
Earth’s orbit around the Sun
Tilt of Earth’s spin axis
Moon’s orbit around the Earth
… as viewed by an extraterrestrial !
Earth’s rotation
• Responsible for our familiar calendar “day”.
• Period (of rotation) = 24 hours
= (24 hours)x(60 min/hr)x(60s/min) =86,400 s
• Astronomers refer to this 24 hour period as a mean solar
day (§2-7), implying that this time period is measured
with respect to the Sun’s position on the sky.
• A sidereal day (period of rotation measured with respect
to the stars – see Box 2-2) is slightly shorter; it is shorter
by approximately 4 minutes.
• The number of sidereal days in a year is precisely one
more than the number of mean solar days in a year!
Earth’s rotation
• Responsible for our familiar calendar “day”.
• Period (of rotation) = 24 hours
= (24 hours)x(60 min/hr)x(60s/min) =86,400 s
• Astronomers refer to this 24 hour period as a mean solar
day (§2-7), implying that this time period is measured
with respect to the Sun’s position on the sky.
• A sidereal day (period of rotation measured with respect
to the stars – see Box 2-2) is slightly shorter; it is shorter
by approximately 4 minutes.
• The number of sidereal days in a year is precisely one
more than the number of mean solar days in a year!
Earth’s rotation
• Responsible for our familiar calendar “day”.
• Period (of rotation) = 24 hours
= (24 hours)x(60 min/hr)x(60s/min) =86,400 s
• Astronomers refer to this 24 hour period as a mean solar
day (§2-7), implying that this time period is measured
with respect to the Sun’s position on the sky.
• A sidereal day (period of rotation measured with respect
to the stars – see Box 2-2) is slightly shorter; it is shorter
by approximately 4 minutes.
• The number of sidereal days in a year is precisely one
more than the number of mean solar days in a year!
Earth’s rotation
• Responsible for our familiar calendar “day”.
• Period (of rotation) = 24 hours
= (24 hours)x(60 min/hr)x(60s/min) =86,400 s
• Astronomers refer to this 24 hour period as a mean solar
day (§2-7), implying that this time period is measured
with respect to the Sun’s position on the sky.
• A sidereal day (period of rotation measured with respect
to the stars – see Box 2-2) is slightly shorter; it is shorter
by approximately 4 minutes.
• The number of sidereal days in a year is precisely one
more than the number of mean solar days in a year!
Earth’s orbit around the Sun
• Responsible for our familiar calendar “year”.
• Period (of orbit) = 3.155815 x 107 s = 365.2564 mean solar
days (§2-8).
• Orbit defines a geometric plane that is referred to as the
ecliptic plane (§2-5).
• Earth’s orbit is not exactly circular; geometrically, it is an
ellipse whose eccentricity is e = 0.017 (Appendix 1).
• Because its orbit is and ellipse rather than a perfect
circle, the Earth is slightly farther from the Sun in July
than it is in January (Fig. 2-22). But this relatively small
distance variation is not responsible for Earth’s seasons.
Earth’s orbit around the Sun
• Responsible for our familiar calendar “year”.
• Period (of orbit) = 3.155815 x 107 s = 365.2564 mean solar
days (§2-8).
• Orbit defines a geometric plane that is referred to as the
ecliptic plane (§2-5).
• Earth’s orbit is not exactly circular; geometrically, it is an
ellipse whose eccentricity is e = 0.017 (Appendix 1).
• Because its orbit is and ellipse rather than a perfect
circle, the Earth is slightly farther from the Sun in July
than it is in January (Fig. 2-22). But this relatively small
distance variation is not responsible for Earth’s seasons.
Earth’s orbit around the Sun
• Responsible for our familiar calendar “year”.
• Period (of orbit) = 3.155815 x 107 s = 365.2564 mean solar
days (§2-8).
• Orbit defines a geometric plane that is referred to as the
ecliptic plane (§2-5).
• Earth’s orbit is not exactly circular; geometrically, it is an
ellipse whose eccentricity is e = 0.017 (Appendix 1).
• Because its orbit is and ellipse rather than a perfect
circle, the Earth is slightly farther from the Sun in July
than it is in January (Fig. 2-22). But this relatively small
distance variation is not responsible for Earth’s seasons.
Earth’s orbit around the Sun
• Responsible for our familiar calendar “year”.
• Period (of orbit) = 3.155815 x 107 s = 365.2564 mean solar
days (§2-8).
• Orbit defines a geometric plane that is referred to as the
ecliptic plane (§2-5).
• Earth’s orbit is not exactly circular; geometrically, it is an
ellipse whose eccentricity is e = 0.017 (Appendix 1).
• Because its orbit is and ellipse rather than a perfect
circle, the Earth is slightly farther from the Sun in July
than it is in January (Fig. 2-22). But this relatively small
distance variation is not responsible for Earth’s seasons.
Tilt of Earth’s spin axis
• Responsible for Earth’s seasons (§2-5)
• Tilt of 23½° measured with respect to an axis that is
exactly perpendicular to the ecliptic plane.
• Spin axis points to a fixed location on the “celestial
sphere” (§2-4); this also corresponds very closely to the
position of the north star (Polaris) on the sky.
• This “fixed location” is not actually permanently fixed;
over a period of 25,800 years, precession of the Earth’s
spin axis (§2-5) causes the “true north” location to slowly
trace out a circle in the sky whose angular radius is
23½°.
Tilt of Earth’s spin axis
• Responsible for Earth’s seasons (§2-5)
• Tilt of 23½° measured with respect to an axis that is
exactly perpendicular to the ecliptic plane.
• Spin axis points to a fixed location on the “celestial
sphere” (§2-4); this also corresponds very closely to the
position of the north star (Polaris) on the sky.
• This “fixed location” is not actually permanently fixed;
over a period of 25,800 years, precession of the Earth’s
spin axis (§2-5) causes the “true north” location to slowly
trace out a circle in the sky whose angular radius is
23½°.
Tilt of Earth’s spin axis
• Responsible for Earth’s seasons (§2-5)
• Tilt of 23½° measured with respect to an axis that is
exactly perpendicular to the ecliptic plane.
• Spin axis points to a fixed location on the “celestial
sphere” (§2-4); this also corresponds very closely to the
position of the north star (Polaris) on the sky.
• This “fixed location” is not actually permanently fixed;
over a period of 25,800 years, precession of the Earth’s
spin axis (§2-5) causes the “true north” location to slowly
trace out a circle in the sky whose angular radius is
23½°.
Moon’s orbit around the Earth
• Responsible for our familiar calendar month.
• Period (of orbit) = 2.36 x 106 s = 27.32 days (Appendix 3).
• Moon’s orbital plane does not coincide with the ecliptic
plane; it is inclined by approximately 8° to the ecliptic
(§2-6).
• Much more about the Moon’s orbit in Chapter 3!
More Details on …
•
•
•
•
Earth’s rotation
Earth’s orbit around the Sun
Tilt of Earth’s spin axis
Moon’s orbit around the Earth
… as viewed by humans (and other
animals) living on Earth.
Rotation of Earth
• Imagine that the Sun and the Moon are
completely dark so that all we see in the
sky (24 hours per “day”) are stars.
• What would “star trails” look like as viewed
from various locations on Earth?
• Example #1: You live on the North Pole
• Example #2: You live on the Equator
• Example #3: You live at other latitudes.
Star Trails …
… looking west!
Rotation of Earth
• Now turn on the Sun, but VERY faintly.
• In a single day, what does the Sun’s “star trail”
look like? In other words, what is the Sun’s
trajectory across the sky in a single day?
• ANSWER: Sun appears to be in the same
direction in the sky as some star (it has a very
specific “celestial sphere coordinate location”)
and its trajectory across the sky will be identical
to that particular star’s “star trail.”
Rotation of Earth
• Now turn on the Sun, but VERY faintly.
• In a single day, what does the Sun’s “star trail”
look like? In other words, what is the Sun’s
trajectory across the sky in a single day?
• ANSWER: Sun appears to be in the same
direction in the sky as some star (it has a very
specific “celestial sphere coordinate location”)
and its trajectory across the sky will be identical
to that particular star’s “star trail.”
… looking west!
… looking west!
… looking west!
… looking west!
July 19; taken at 69º north latitude in northeast Alaska.
Seasons …