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Preceptors and Times
REMEMBER HW DUE EVERY MONDAY NIGHT
Ch. 1 and Ch 2 HW will be due this Mon. Sept 6
Last time: 2.1. Patterns in The Sky: Stars and constellations, TODAY:
celestial coordinates2.2 Seasons,…Precession2.3. The Moon and Eclipses
Lunar Phases and Eclipses2.4 Ancient Mystery of the Planets:
Apparent Retrograde motion of planets
ParallaxCh 3 (Histrory of Astronomy)
Class 3: End of Ch 2 and Ch. 3
2.2 Seasons
• What is the cause of the seasons on Earth?
Annual Motion• The Earth’s axis is tilted 23.5° from being
perpendicular to the ecliptic plane.• Therefore, the celestial equator is tilted 23.5°
to the ecliptic.• As seen from Earth, the Sun spends 6 months
north of the celestial equator and 6 months south of the celestial equator.
• Seasons are caused by the Earth’s axis tilt, not the distance from the Earth to the Sun!
Annual Motionecliptic
the apparent path of the Sun through the sky(also the plane of Earth’s orbit)
equinoxwhere the ecliptic (the Sun) intersects the celestial equator
solsticewhere the ecliptic (the Sun) is farthest from the celestial equator
zodiacthe constellations which lie along the ecliptic
The Cause of the Seasons
Coordinates on the Celestial Sphere (not in book)
• Latitude: position north or south of equator• Longitude: position east or west of prime
meridian (runs through Greenwich, England)
• Declination: position north or south of celestial equator (in degrees)
• Right Ascension: distance (in hours, 0 to 23h 59 min.) East of vernal equinox (where the sun crosses the celestial equator going North)
What point on the celestial sphere is defined by a Right Ascension of zero hours and a Declination of zero degrees?
Question 1
What point on the celestial sphere is defined by a Right Ascension of zero hours and a Declination of zero degrees?
The vernal (spring) equinox
Question 1
What point on the celestial sphere is defined by a Right Ascension of 12 hours and a Declination of zero degrees?
Question 2
What point on the celestial sphere is defined by a Right Ascension of 12 hours and a Declination of zero degrees?
The autumnal (fall) equinox
Question 2
What is an equinox?
Question 3
What is an equinox?When the Sun (the ecliptic) crosses the celestial equator
Question 3
What point on the celestial sphere is defined by being on the ecliptic at a Declination of +23.5 degrees?
Question 4
What point on the celestial sphere is defined by being on the ecliptic at a Declination of +23.5 degrees?
The Summer Solstice
Question 4
What point on the celestial sphere is defined by being on the ecliptic at a Declination of minus 23.5 degrees (i.e. 23.5 degrees South)?
Question 5
What point on the celestial sphere is defined by being on the ecliptic at a Declination of minus 23.5 degrees (i.e. 23.5 degrees South)?
The Winter Solstice
Question 5
Axis tilt causes uneven heating by sunlight throughout the year.
When is summer?(in the Northern Hemisphere)
• The solstice which occurs around June 21 is considered the first day of summer.
• However, it takes time for the more direct sunlight to heat up the land and water.
• Therefore, July & August are typically hotter than June.
2.2 (Cont) Precession
• What is the Earth’s cycle of precession?
Thought Question What is the arrow pointing to?A. The zenithB. The north celestial poleC. The celestial equator
Thought Question What is the arrow pointing to?A. The zenithB. The north celestial poleC. The celestial equator
Precession• The Earth’s axis precesses (wobbles) like a top, once about every 26,000 years.• Precession changes the positions in the sky of the celestial poles and the equinoxes. Polaris won't always be the north star. However the tilt in the axis is the same (23.5 degrees) as the Earth’s axis precesses
A. Will Polaris be the North star in another 13,000 years?
Question
B. Will Polaris be the North star in another 26,000 years?
Question
2.3 The Moon, Our Constant Companion
• Why do we see phases of the Moon?• What conditions are necessary for an eclipse?
Lunar MotionPhases of the Moon’s 29.5 day cycle• new• crescent• first quarter• gibbous• full• gibbous• last quarter• crescent
waxing
waning
Lunar phases and the month
• A cycle of lunar phases is approximately 30 days and is the reason why we divide the year in 12 months
• Why do lunar phases occur on different calendar dates from one year to the next?
Lunar phases and the month
• A cycle of lunar phases is approximately one month and is the reason why we divide the year in 12 months
• Why do lunar phases occur on different calendar dates from one year to the next?
• Because one year (365 days) is not exactly 12 times the period of the Moon’s phases (its actually 12.4 times).
Why do we see phases?
• Half the Moon illuminated by Sun and half dark
• We see some combination of the bright and dark faces
Moon’s position at sunset for waxing phases
(numbers are days since new Moon)
Phases of the MoonDid you see the Moon last night? Why?
What is the moon’s phase today?
When does it rise and set?
Why do we see the same side on our Moon?
Rotation period = orbital period
Eclipses
• The Earth & Moon cast shadows.
• When either passes through the other’s shadow, we have an eclipse.
• Why don’t we have an eclipse every full & new Moon?
EclipsesWhen the Moon’s orbit intersects the ecliptic (node):
at new moon solar eclipse
at full moon• everyone on the nighttime side of Earth can see it
lunar eclipse
you must be in Moon’s shadow to see it• within umbra: total solar eclipse• within penumbra: partial solar eclipse
Solar Eclipse
What will be the Right Ascension and Declination of the Moon during a total solar eclipse on September 21?
Question
What will be the Right Ascension and Declination of the Moon during a total solar eclipse on September 21?
Hint: where is the Sun on Sept 21?
Question
What will be the Right Ascension and Declination of the Moon during a total solar eclipse on September 21?
Hint: where is the Sun on Sept 21?Right Ascension = 12 hDeclination = 0 degrees
Question
What will be the Right Ascension and Declination of the Moon during a total solar eclipse on September 21?
Hint: where is the Sun on Sept 21?Right Ascension = 12 hDeclination = 0 degreesSo where will the Moon be?
Question
What will be the Right Ascension and Declination of the Moon during a total solar eclipse on September 21?
Hint: where is the Sun on Sept 21?Right Ascension = 12 hDeclination = 0 degreesSo where will the Moon be?Same place because there is a total solar
eclipse
Question
Lunar Eclipse
What have we learned? • Why do we see phases of the Moon?
• At any time, half the Moon is illuminated by the Sun and half is in darkness. The face of the Moon that we see is some combination of these two portions, determined by the relative locations of the Sun, Earth, and Moon.
• What conditions are necessary for an eclipse?• An eclipse can occur only when the nodes of the
Moon’s orbit are nearly aligned with the Earth and the Sun. When this condition is met, we can get a solar eclipse at new moon and a lunar eclipse at full moon.
