AS101 Module 1

Cosmic Calendar

- scale on which we compress the history of the universe into 1 year, created by Carl Sagan
- it has been about 13.8 billion years since the big bang, the instant the universe commenced
- in this scale, the Milky Way Galaxy starts to coalesce (form) in late February or early March, our solar system starts being built around mid-August, and by the end of September primitive life exists on Earth, mid-december complex living structures such as invertebrate life formed, December 25 when dinosaurs roamed Earth and finally December 31, is when all of recorded history occurred and even then not until much later in the day—within the last 30 seconds

zenith

point in the sky directly overhead no matter where you are on Earth

celestial equator

- imaginary circle created by extending Earth's equator into space (celestial sphere)
- always cuts your horizon due east and arcs across the sky falling into the horizon due west
- passes through your zenith ending at the horizon due west
- imaginary sphere of very large radius surrounding Earth to which the planets, stars, Sun, and Moon seem to be attached

Meridian

- arc drawn on a map between the North and South poles
- Arc line going from due north (north celestial pole), through your zenith and finishing due south (south celestial pole)
- cuts the celestial equator at a right angle

North Celestial Pole (NCP)

- point on the celestial sphere directly above Earth's North Pole and northern point of the apparent rotation of the celestial sphere
> same concept applies for South Celestial Pole

angular size

the angle an object appears to span in your field of view
- moon and sun = 1/2 of a degree or 0.5°
- Angular degrees are subdivided into arc minutes (60′ in one degree); an arc minute is further subdivided into 60 arc seconds (″)

Stellar Motions

- Statistically the stars that have larger motions are nearer
- By measuring the motions of a large number of stars, we can estimate their average distance from their average motion
- motion of the stars at night depends where on the Earth's surface you are located
- night-time pattern of stars changes little from one night to the next. But gradually, the star patterns shift as the Earth orbits the Sun

solar system

the sun, its planets, asteroid and comets
- Sun is about 110 times larger in diameter than the Earth
- the sun is a star -> self-luminous ball of hot gas that generates its own energy
> about 5 billion years. It is assumed that it will last about another 5 billion years or so
- earth is 13 000 kilometres in diameter
- Earth, Venus and Mercury are planets—small, nonluminous bodies that shine by reflecting sunlight
> Venus is about the size of Earth, and Mercury is a bit larger than Earth's moon (smaller than Earth)
- other major planets in our solar system—Mercury, Venus, Mars, Jupiter, Saturn, Uranus, and Neptune—orbit our Sun in ellipses that are nearly circular

cycle of the sun

- over a timeframe of several weeks, it moves eastward relative to the distant stars, all the while moving from east to west each day
- due to the fact that the Earth moves around the Sun on its annual orbit
- gradually it is moving either closer to your zenith or further away from it
> between the summer solstice and the winter solstice the Sun's path is moving slowly southward away from your zenith and from the winter to summer solstice its path is moving just a little more northward each day

movement of earth

- as it moves along the ecliptic path its rotational axis (north-south axis), on which it makes one revolution each day, is tipped to the ecliptic plane at a constant angle of 23.4° resulting in seasons
> tipped rotational axis always points to the same point in space as the Earth revolves around the Sun
- earth ROTATES around itself one daily and REVOLVED around the sun once per year
- seasons are a result of the changes in the amount of solar energy received by the northern and southern hemispheres
summer solstice = sunlight strikes the Earth's surface in Canada at a much steeper angle and the light spreads out less than when it strikes the ground therefore it warms the surface more and we have summer
> From the winter to summer solstice its path is moving just a little more northward each day which makes the angle shallower

Tropic of Cancer

23.4 degrees north of the equator
- if you were standing at this latitude on Earth on June 21 (summer solstice) the Sun would rise north of due east, cross your meridian at your zenith and set north of due west

Tropic of Capricorn

a line of latitude about 23.4 degrees South of the equator
- Sun's path is south of the celestial equator reaching its most southward path on December 21 (winter solstice)
> @ the equator on this day the sun rises south of due east by about 23.5 degrees and cuts across the sky remaining parallel to the celestial equator and sets south of due west by about 23.5 degrees

movement of the moon

takes about a month to circle the Earth
- orbit is not precisely circular, its distance from Earth varies
- it goes through phases of reflected sunlight and as it does the times of rising and setting change throughout the cycle
- as the Moon orbits the Earth the Earth moves in its orbit around the Sun, naturally taking the Moon with it
- only ever see one side of it because it is tidally or gravitationally locked to Earth resulting in a motion known as synchronous rotation
> backside of the Moon that gets illuminated so we can't see the Moon at all

synodic month

29.5 days
- amount of time from one full moon to the next
- lunar phase's cycle, which is about 29.5 solar days, corresponding roughly to the average length of a month
- also called the orbital period of the moon

sidereal month

- just over 27 days
- time it takes for one revolution relative to the stars
- different from the synodic month because of Earth's continuous motion around the Sun

sidereal year

- time required for Earth to complete exactly one orbit around the Sun, relative to the stars
- about 365.25 days

tropical year

- time between successive spring (or autumnal) equinoxes
- sidereal year is shorter than this by about 20 minutes, and the difference is due to the precession of Earth's rotation

apparent solar time

- time as measured by the position of the Sun in the sky relative to our local meridian (the time that would be indicated by a sundial)
> such that noon occurs when the Sun crosses the meridian
- each solar day differs from 24 hours by a slight amount because Earth's orbit is not perfectly circular and because of Earth's 23.4-degree tilt
> varies with longitude (because of Earth's spin on its axis)
- average solar day is the more important concept and the one used to keep track of time as it is divided into 24 different time zones so within each time zone the time is exactly the same

Phases of the moon

new = exactly between the Earth and Sun
> changing shape as it passes through its cycle of phases is produced by sunlight illuminating different parts of the side that you can see
> we can see the moon during the day as it rises early in the morning (at 6 am) and sets in the evening (at 6 pm)
waxing crescent = tiny sliver as moon starts to come around
first quarter = half moon
> rise at 6pm and set at 12am
waxing gibbous = almost all of the moon is light up on its way to full
full moon = moon is on the other side of the earth so it is fully lit up
> moon is in line with the earth and the sun
waning gibbous = almost all of the moon is light up on its way down from full
third quarter = half moon (as light begins to fade)
> rise at midnight and set at noon
waning crescent = tiny sliver as moon finishes getting down

eclipse

- only happens when the sun, earth and moon are in a straight line AND the phase of the moon must be either new or full
- this should happen monthly with every new (solar) and full (lunar) moon however this doesn't happen because the Moon's orbit is inclined at about 5° to the ecliptic plane so the Moon spends most of its time either above or below the plane
> Only when it is crossing through this plane, at points called nodes coverage possible
- occurs in a cycle of approx 173 days, slightly less than exactly 6 months
> cycle of both lunar and solar occurs over a period of about 18 years 11.3 days, a timeframe known as the saros cycle

lunar eclipse

darkening of the Moon when it moves through Earth's shadow
- only at full moon when the Moon moves through the shadow of Earth
- Earth blocks out the Sun's light at midnight for a period of time
- occurs up to 3 times a year and can be seen throughout the entire night
- can occur only when the Sun, Earth, and Moon lie along a straight line in that order
- the nodes of the Moon's orbit must be nearly aligned with the Sun and Earth and the phase of the Moon must be full
3 types:
1. penumbral -> most common, sunlight is only partially blocked and the result is that the Moon darkens only slightly
2. Partial -> part of the moon passes through the umbra while the rest passes through the penumbra, resulting in part of the Moon being darkened completely but rest is only slightly darkened
3. Total -> Moon passes entirely through the umbra resulting in the moon being completely dark during the eclipse

astronomical unit

- unit of length used for distances within the solar system
- roughly the mean Earth-Sun distance
- 1.5 x 10^8 km / 1.5 x 10^11 m / 150 million km

Light Year (LY)

the distance light travels in one year
- roughly 10^13⁢km, or 63 000 AU

galaxy

great cloud of stars, gas, and dust bound together by the combined gravity of all the matter
ex- the one that Earth is in is called the Milky Way which has graceful spiral arms winding outward through the disk
> 80 000 ly in diameter and contains over 100 billion stars

solar eclipse

- Occurs when a new moon passes directly between the Sun and Earth and casts a shadow over part of Earth
- moon blocks out the sunlight at high noon for a period of time
- when the Earth passes through the Moon's shadow or when the Sun, Moon and Earth are lined up in that order
- cannot look directly at the sun when this happens and it happens for approx 2 hours
- Moon casts a shadow that reaches all the way to part of Earth's surface, a shadow with two parts
3 types:
1. Total -> when the Moon is relatively close to the Earth in its orbit and the Moon's umbra touches a small area of the Earth's surface
> people within this area sees the sun blocked out aka totality view
2. Partial -> surrounding the small area of totality lies a larger area falling inside the Moon's penumbral shadow — anyone within this area sees the Sun only partially blocked out
3. Annular -> occurs when the Moon is relatively far from Earth and the Moon's umbra may not reach the Earth surface at all
> anyone within the small area behind the umbra will see all of the Sun blocked out except a ring of sunlight surrounding the Moon's disk

order of the planets from the closest one to the Sun (Mercury) to the furthest one (Neptune)

Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune
*My Very Educated Mother Just Served Us Nachos
- their movements generally follow the motions of the Sun and Moon in that they also move eastward relative to the stars

retrograde motion

The apparent motion of the planets when they appear to move backwards (westward) with respect to the stars from the direction that they move ordinarily.
- occurs because planets do not move at a constant rate due to the differences in orbit size between different planet
- inner planet moves faster in its orbit. As the inner planet catches up to and passes the outer, slower-moving planet, the outer planet "appears" to move backwards in the sky
- readily observable for so-called outer planets beginning with Mars where it is most obvious
> also appears for the inner planets (Venus and Mars) but it's almost impossible to observe because this motion takes place when Earth, planet and Sun are all aligned so the Sun's glare prevents us from making the observation

Stellar Parallax

- apparent shift in the position of a star when viewed from different locations
- how a star looks different when we first view it when the Earth is at one extreme of its orbit around the Sun, and second when the Earth is at the opposite extreme six months later
> the nearby star (object) appears to shift laterally against the background of stars behind it
- allows us to measure distances to nearby stars AND also provides direct evidence that the Earth really does revolve around the Sun
- not detectable with the naked eye because the amount of shift in a nearby star's position relative to the far-away background of stars is too small

umbra

the region of total shadow during an eclipse

Geocentric Universe

- the idea that planets revolved in perfect circular orbits around the earth in the universe
- A model of the universe with Earth at the centre, such as the Ptolemaic universe
- Aristotles universe
- heavenly objects moved on circular paths at constant speed, with Earth motionless at the centre of the universe
- Ptolemy weakened the first principles of Aristotle by moving Earth a little off-centre in the model and inventing a way to slightly vary the planets' speeds
> better match to the observed motions and could handle the complicated retrograde motion of the planets

magnitude scale

- astronomical brightness scale; the larger the number, the fainter the star.
- brightest were called first-magnitude stars, the next brightest were called second-magnitude, and so on downward to sixth-magnitude stars
> first-magnitude are about 100x brighter than sixth-magnitude
- The Greek astronomer Hipparchus (190-120 BCE) is believed to be the first to catalogue stars by their apparent brightness
- scientifically accurate we refer to flux (measure of the light energy from a star that hits one square metre in one second) instead of going based on the naked eye

magnitude scale limitations

1. Some stars are so bright the magnitude scale must extend into negative numbers
2. With a telescope you can find stars much fainter than the limit for your unaided eyes, so the system has also been extended to numbers larger than sixth magnitude to include faint stars
3. Although some stars emit large amounts of infrared or ultraviolet light, those types of radiation are invisible to human eyes

Precession

- slow motion of Earth's axis that traces out a cone over a period of 26,000 years
- celestial poles and equator were slowly moving relative to the stars
- Earth has a slight bulge around its middle, and the gravity of the Sun and Moon both pull on this bulge, tending to twist Earth's axis upright relative to its orbit.
> combination of these forces and Earth's rotation causes Earth's axis to precess in a slow circular sweep
> traces an imaginary cone over this time frame

movement of the sun

- Through the year, it moves eastward among the stars following a line called the ecliptic
- it appears to go once around the sky in a year
- You don't notice this motion because you cannot see the stars in the daytime, but the apparent motion caused by a real motion of Earth causes the seasons

Penumbra

- part of a shadow surrounding the darkest part
- portion of a shadow that is only partially shaded during an eclipse

saros cycle

period over which the basic pattern of eclipses repeats, which is about 18 years 11 1/3 days

solar day

- From noontime to the next noontime; for Earth it is 24 hours.
- average length of time between successive passes of the Sun across the local meridian
- varies slightly throughout a year
- Earth moves about 1 degree per day around its orbit

sidereal day

- time for one complete rotation of the earth relative to a particular star, about 4 minutes shorter than a mean solar day
- the time it takes for any star to make successive passes across the local meridian
- during a solar day, Earth has travelled along its orbit around the Sun and needs a little more time to rotate before the Sun crosses the meridian which is why the solar day is a bit longer (earth is revolving around the sun and rotating as well vs earth is just rotating in relation to the star)

declination

- measure of how far north or south an object is from the celestial equator
- latitude for the sky
- expressed in degrees, arcminutes, and arcseconds north (+) or south (-) of the celestial equator
> refer to the size of the angle between a line from the Earth's equator and the line to the particular location on the celestial sphere, both starting at the center of the earth

right ascension

- the angular distance east - west along the celestial equator
- longitude for the sky
- expressed in hours, minutes, and seconds
> Earth completes one turn in 24 hours, so the celestial sphere appears to take 24 hours to complete one turn around Earth
- zero = the longitudinal line that runs through the spring equinox and then this is then measured going east from this longitudinal line
- unlike the stars which don't move much (too far from earth), the Sun moves along the ecliptic throughout the year, traveling through the complete 24 hour zones over the calendar year

Gregorian calendar

- revision of the Julian calendar by Pope Gregory XIII; currently used in most of the world
- every 4 years we have a leap year and additionally, set the spring equinox to March 21 and then adjusted the leap day schedule such that each century year (normally a leap year) would be skipped as a leap year unless that year was divisible by 400
- this helps us never get behind/ahead of the calendar due to the fact that the tropic year is 11 minutes short of 365.25 days