Homework 1 has been posted It is due
Homework #1 has been posted It is due by Friday, Jan. 28, 4 p. m. Answers can be submitted at any time
Look at Applets portion of class website. Applets linked to chapters 1, 2 and S 1 are relevant to the current discussion
Understanding changes in the sky
We have been examining the daily motion of the stars through the local sky due to the Earth’s rotation. What about the Sun’s apparent motion in the sky?
The Earth makes a complete revolution about the Sun in one year (orbit shown in perspective; it is actually very close to being circular in shape)
Ø The Sun moves east to west, full circuit around the sky, each “day” (result of Earth’s rotation) Ø Relative to the stars, the Sun moves slowly eastward each day (~ 1 degree/day): full circuit around Celestial Sphere in one “year” (result of Earth’s orbit)
Ø The Sun moves east to west, full circuit around the sky, each “day” (diurnal motion - result of Earth’s rotation) Ø Relative to the stars, the Sun moves slowly eastward each day (~ 1 degree/day): full circuit around Celestial Sphere in one “year” (result of Earth’s orbit) Ø Path of Sun on Celestial Sphere due to Earth’s orbital is a great circle called “Ecliptic”. intersection of the Earth’s plane (Ecliptic plane) and the Celestial sphere. motion the This is the orbital
Plane of the Celestial Equator (blue) and the Ecliptic Plane (yellow) are inclined by 23 ½ degrees to each other
Today we divide the celestial sphere into 88 regions, referred to as constellations (in a manner similar to dividing the Earth’s surface into countries).
The constellations along the ecliptic are called the zodiac (see applet)
Two measures of the Earth’s rotational period Solar day: Rotational period relative to the sun (successive sun crossings of the meridian). This is the common measure of the day. Sidereal Day: Rotational period relative to the stars (successive star crossings of the meridian, 23 h 56 m)
The Vernal Equinox serves as the origin for measuring Right Ascension (R. A. = celestial longitude)
We tell time by the location of the Sun in the sky, relative to our meridian
Plane of observer’s meridian Locationof of observer Eastern horizon The Earth, as seen from above the North Pole Western horizon Rotation direction of the Earth North Pole The meridian divides the sky into eastern and western halves
Planes of local meridians at different locations The meridian is defined by the location of the observer on the Earth North Pole
Planes of local meridians at different locations The horizon is also defined by the location of the observer on the Earth North Pole Planes defining local horizon
Planes of local meridians at different locations The meridian is defined by the location of the observer on the Earth North Pole The time of day is determined by the position of the sun relative to the observer’s meridian.
Time of day determined by the position of sun relative to the observer’s meridian. Noon: Sun on meridian Midnight: 6 pm: 6 am: Sun 180 degrees from meridian Sun 90 degrees west of meridian Sun 90 degrees east of meridian
(displayed clock times are approximate) (small arrows “ “ point toward local meridian) Sunset (6 pm) 3 pm 9 pm Noon Midnight Earth 3 am 9 am Sunrise (6 am)
Where is the local time noon? blue red orange yellow Earth green
Where is the local time noon? blue red orange yellow Earth green
Where is the sun setting? blue red orange yellow Earth green
Where is the sun setting? blue red orange yellow Earth green
Where is it midnight? blue red orange yellow Earth green
Where is it midnight? blue red orange yellow Earth green
Where is the local time 6 am (sunrise)? blue red orange yellow Earth green
Where is the local time 6 am (sunrise)? blue red orange yellow Earth green
Time is determined by where you are on a sphere, relative to where the sun is located. What if we view a sphere from different vantage points not on the sphere?
Motions and Phases of the Moon The Moon’s orbit around the Earth
Two measures of the Moon’s orbital period Synodic Period: Period relative to the sun (approx. 29 ½ days) – determines phases of the moon Sidereal Period: orbital period relative to the stars (approx. 27. 3 days)
Lunar Rotation: No rotation - never seen in astronomical bodies Prograde rotation – rotation is in same sense as orbit Synchronous – rotation period is the same as its orbit period: same side of Moon always faces Earth
The Lunar Phases is determined by our viewing orientation of the Moon relative to its sunlit half. Thus the relative positions of the Sun, Earth, and Moon determine the phase.
An interactive tool and tutorial in Time, the phases of the Moon, and the location in the sky where the Moon is located at different times and phases: LUNAR PHASES WEB TOOL http: //www. calvin. edu/~lmolnar/moon Visit this site and become familiar with its contents
Viewed from “north” (far above Earth’s north pole)
See also
1. Suppose you lived on the Moon. If the Moon is in its first quarter phase, what would the phase of the Earth be as seen from the Moon? 2. What time (on Earth) does the first quarter moon rise? Set? In which direction does it rise? 3. At sunrise (in Bloomington) you happen to notice that the Moon is on your meridian. What is its phase?
Suppose you lived on the Moon. If the Moon is in its first quarter phase, what would the phase of the Earth be as seen from the Moon? ”
Suppose you lived on the Moon. If the Moon is in its first quarter phase, what would the phase of the Earth be as seen from the Moon? ” First quarter Moon will occur at the position shown. Realizing that the left side of the Earth (as shown in the figure) is in shadow, the Earth as seen from the Moon will appear in its third quarter.
What time (on Earth) does the first quarter moon rise? Set? In which direction does it rise? setting
What time (on Earth) does the first quarter moon rise? Set? In which direction does it rise? The first quarter moon is located at position #3 in the figure. It will be on the eastern horizon on the earth when it rises and the western horizon when it sets (recall that the Earth is rotating counter-clockwise). These positions are shown on the diagram. setting
At sunrise (in Bloomington) you happen to notice that the Moon is on your meridian. What is its phase? Sun is rising here
At sunrise (in Bloomington) you happen to notice that the Moon is on your meridian. What is its phase? Sunrise is occurring at position 7. If the moon is on your meridian, it must be at location 7. This corresponds to 3 rd quarter. Sun is rising here
Understanding the visible sky and motions within the sky is easily accomplished Ø using the concept of Celestial Sphere Ø understanding the rotational & orbital motions of the Earth
- Slides: 47
