PHY 1060 Lecture Notes - Lecture 2: Equinox, Escape Velocity, Lunar Phase
Astronomy Module 2
Chapter 2: Patterns in the Sky
• Our focus:
o Earth’s daily rotation
o Earth’s annual orbit
o The Moon’s monthly orbit
o Consequences of the relationship/alignment of Earth, the Moon, and the Sun
(phases of the Moon, eclipses).
• Constellations: arbitrary patterns of stars in the sky
o Culturally important products of human imagination
▪ Not physically related and far from each other
▪ Only appear to be at the same distances
o Ancient cultures built structures to study them and other astronomical phenomena
▪ The zodiac are constellations that the sun passed through
• As viewed from above the North Pole, Earth rotates counterclockwise on its axis
o One rotation takes 24 hours
• Celestial sphere: projection of Earth’s axes and equator onto space
o A useful fiction
▪ Imagined to be much larger than the Earth
o Points on the sphere correspond to directions, not distances
o Ecliptic line: path of the sun’s apparent motion in the Earth’s sky
o The sun rises in the east and sets in the west, so the earth is rotating towards the
east
• At Earth’s north pole, you would see half the celestial sphere
o The north celestial pole is directly overhead
▪ Northern star
o Stars rotate counterclockwise in 24 hours
o No star rises or sets: all are circumpolar
▪ Circumpolar: gives the illusion that stars are rising or setting
• At the equator observers can see the whole celestial sphere as it rotates
o The celestial poles are on the northern and southern horizons
• The average distance of Earth to the Sun is called the astronomical unit (AU)
o 1 AU = 150 mil km
o Earth revolves counterclockwise around the Sun as viewed from above
• The Sun’s motion on the ecliptic reflects Earth’s orbit around the Sun
o Earth revolves around the sun once a year
o As Earth moves, the sun is seen against different constellations
▪ The zodiac
• Earth’s axis is not perpendicular to the ecliptic plane
o Instead it is at an angle of 23.5 degrees
o This why there are seasons
• The angle of sunlight is closer to perpendicular in summer
o Energy is more concentrated
o The southern hemisphere is opposite the northern hemisphere
o If the ecliptic were aligned with the celestial equator, we would not have seasons
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• Special days of the year
o Summer solstice: sun farthest north
▪ Longest day
o Autumnal equinox: sun on the equator moving southward
▪ Day and night are of equal length
o Winter solstice: sun farthest south
▪ Shortest day
o Vernal equinox: sun on the equator, moving northward
▪ Day and night are of equal length
• Currently, the north celestial pole is near the bright star Polaris
o Earth’s axis wobbles with a period of 26,000 years
o Location of the poles slowly shifts
o Earth wobbles like a top, slowly
o Since the axis shifts, the equator shifts
o Positions of the equinoxes precess as well
• We only see one face of the moon
o Synchronous orbit
▪ Completes one full rotation in one full orbit around earth
o There is no dark side of the moon
▪ Near side and far side
• The moon shines because of reflected sunlight
o Half of the moon is always bright
o The phase is determined by how much of the bright side we see
• Phases of the moon
o New moon: moon between earth and sun
o Waxing crescent
▪ Going from dark to light
o Quarter moon: moon at right angles with earth and the sun
o Waxing gibbous
o Full moon: moon on opposite side of earth from the sun
o Waning crescent and gibbous
▪ Going from light to dark
o In the northern hemisphere, the moon fills in from right to left and fades out from
right to left
• The moon takes 27.3 days to orbit once with respect to the stars (sidereal period)
o Lunar phases repeat every 29.5 days (synodic period)
o The moon’s rotational period equals its sidereal period
• Throughout history, humans have made calendars based on the seasons or the moon’s
cycle of phases
o The number of calendar days (365) does not fit neatly into months (29.5 days) or
years (365.24 days)
o Humans creatively made calendars to deal with this
o Today the Gregorian calendar, which includes leap years to keep it on track, is
used
• Solar eclipses happen at new moon
o Moon passes between Earth and the Sun
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