Lesson 2 Flashcards
Constellation
- Region of the sky with well-defined borders
- 88 official constellations
Celestial sphere
- Imaginary sphere that surrounds Earth, where all the stars lie
North celestial pole
point directly over earth’s north pole
South celestial pole
point directly over earth’s south pole
Celestial equator
projection of earth’s equator into space
Ecliptic
Path the Sun follows as it appears to circle around the celestial sphere once a year (crosses celestial equator at 23.5 degrees; because that is the tilt)
Local Sky
sky as seen from wherever you stand
Horizon
boundary between Earth and sky
Zenith
Point directly overhead
Meridian
imaginary half circle from our horizon South to North
Azimuth
degrees clockwise from due north - direction along horizon
Altitude
degrees above horizon
Angular size
Angle an object appears to span in your field of view
Moon and Sun are both 0.5 degrees
Angular distance
angle that appears to separate two objects
Arcminutes and arcseconds
- Arcminutes: divide each degree into 60 arcminutes
- Arcseconds: divide each arcminute by 60 arcseconds
Rotation of stars
- stars near north celestial pole are circumpolar (they remain where they are, just rotota)
- Stars near south celestial pole never rise above horizon
- all other stars have daily circles that are partly above horizon and partly below (appear to rise in East and set in West)
Latitude and longitude
- Latitude measures north-south position
- Longitude measures east-west position
- Latitude affects how we see constellations
Zodiac
- the constellations along the ecliptic
- 13 constellations
Why seasons?
- the tilt of Earth’s axis causes sunlight to fall differently on Earth at different times of year
- North Hemisphere: tipped toward sun in June and away from sun in December
- Southern Hemisphere: tipped toward sun in December and away from sun in June
June Solstice
- Summer solstice in North
- June 21st
- North Hemisphere tipped most directly toward the Sun - most direct sunlight
December Solstice
- Winter solstice in North
- December 21st
- North tipped away; least direct sunlight
March equinox
- Spring/vernal equinox in North
- March 21st
- moment where North goes from tipped away to tipped toward Sun
September equinox
- Fall/autumnal equinox in North
- September 22nd
- moment where North goes from tipped toward to tipped away Sun
Precession
- gradual wobble that alters orientation of Earth’s axis in space
- each cycle takes about 26,000 years
- gradually changes the direction in which the axis points in space (polaris won’t be north star, it’ll be Vega)
- Only changes what we see in space, not seasons (constellations) - so zodiacs change
- Tropical year is actually longer by 20.2 minutes
Lunar Phase
- moons orbit around sun takes 29.5 days (synodic period)
- inclination of lunar orbit to ecliptic plane is 5 degrees
Moon phases: New Moon
New Moon: rises and sets with the sun; can’t see it in night
- moon is between earth and sun
Moon phases: Waxing Crescent
Waxing crescent: rises mid-morning, highest in mid-afternoon and sets late evening; sliver getting bigger
Moon phases: First Quarter
First Quarter: rises noon, highest sunset, sets midnight; looks like a half moon getting bigger
Moon phases: Waxing Gibbous
- Waxing Gibbous: rises mid-afternoon, highest late evening, sets before dawn; looks like three quarter full moon and getting bigger
Moon phases: Full moon
Full moon: rises sunset, highest midnight, sets sunrise; full because sun illuminates it fully
Moon phases: Waning Gibbous
Waning Gibbous: rises late evening, highest before dawn, sets mid-morning; looks three quarter full getting smaller
Moon phases: Third Quarter
Third Quarter: rises midnight, highest sunrise, sets noon; looks half full getting smaller
Moon phases: Waning Crescent
Waning crescent: rises before dawn, highest mid morning, sets mid-afternoon; looks like crescent getting smaller
Synchronous Rotation
We always see same face of Moon
Lunar eclipse
- Earth lies between Sun and Moon, so Earth’s shadow falls on Moon
- Total lunar eclipse: sun earth moon almost exactly lined up (all umbra); red during totality
- Partial lunar eclipse: not quite lined up (part umbra, part penumbra)
- Penumbral lunar eclipse: all penumbra (most common)
Solar Eclipse
- Moon lies between Sun and Earth, so moon’s shadow on Earth
- Total solar eclipse: moon close to earth so umbra covers lots
- Annular solar eclipse: moon is farther, umbra may not reach totally, ring of sunlight seen around moon
- Partial solar eclipse: only part of the sun is blocked from view
Nodes
- where the moon crosses through Earth’s ecliptic plane; happens twice an orbit; only have eclipses when moon is near these nodes and full or new moon
- lunar nodes precess around ecliptic, completing revolution every 18.6 years
Eclipse season
- nodes of the moon’s orbit are closely aligned with the Sun
- lasts <5 weeks; usually one lunar and one solar eclipse
Saros cycle
- eclipses recur in a cycle of ~ 18 years and 11.25 days
Planets easy to see with naked eye
- Mercury, Venus, Mars, Jupiter, and Saturn
Apparent retrograde motion
- occasionally the planets look like they go east to west instead of west to east because Earth is passing other planets
Stellar Parallax
- apparent shift of position of any nearby star against distant background
- more distant the object the smaller it’s stellar parallax
- made it difficult to abandon Earth centered universe because couldn’t detect stellar parallax
Sidereal vs. Solar day
- Sidereal day: Earth goes 360 degrees; orbits the sun once every 366.26 sidereal days
- Solar day: Earth goes from noon to noon; orbits the sun once every 365.26 solar days
- Length of solar day varies throughout the year; because ellipse (moves faster closer to sun) and because earth is tilted (seasons)
Solar day increase
- solar day is increasing 1.4 milliseconds every century
Daytime
- due to refraction and scattering of light of atmosphere, day light earlier than sunrise
- at equator; daytime and nighttime are equal
- but not: daytime and nighttime change depending on solstice
Synodic month vs. Sidereal month:
- Synodic: 29 days 12 hours and 44 minutes
- Sidereal: 27.322 days
