ASTR 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 By 11:30 am, 25 January 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 Rotation Period (solar days) Orbital (sidereal) Period (solar days) Inclination of equator to orbit (degrees) “Moon’s” orbital period (solar days) Earth °27.32 Mars ° Two satellites: & Jupiter ° Thirty-nine satellites! Mercury ½° No satellites  Venus243 (R) ° No satellites  Uranus (R) 30, ° Twenty-seven satellites! Saturn Neptune

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 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) = x 10 7 s = 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 = (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) = x 10 7 s = 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 = (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) = x 10 7 s = 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 = (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) = x 10 7 s = 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 = (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 10 6 s = 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!

July 19; taken at 69º north latitude in northeast Alaska.

Seasons …