observing the sky the birth of astronomy. pre-historic astronomers

Observing the SkyThe Birth of Astronomy

Pre-Historic Astronomers

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Need for Astronomy• Predicting seasons enables Survival

– Migration – Store food (like squirrels)– Plant crops in spring (after last frost?)– Predict seasonal flooding

• Knowing time of day enables Survival– Hide in cave at night (lions, tigers & bears!)

• What else is there to do at night without a light?

Astronomy tells time of year• Which stars are up at night

– Star patterns = Constellations• Orion high in winter

• Cygnus high in summer

• Altitude of sun at noon– High in summer

– Low in winter

• Location of sunrise/sunset– NE/NW in summer

– SE/SW in winter

– E/W on 1st day of spring/fall

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Constellations and Star Maps

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Constellations

• 88 Official Constellations– Examples: Ursa Major, Taurus, ...– Often drawn as “stick figures” (stick figures not

official; may vary)– Official Constellations are regions of the sky (like

states)

• Asterism – a popular name for a group of stars that is not an

official constellation– Examples: The Big Dipper, The Pleiades

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The Celestial Sphere

Earth inside Celestial Sphere

Locations on Celestial Sphere

• Zenith – straight up (overhead)

• Nadir– straight down (beneath your feet)

• Horizon – lowest place you see the sky– Shape: a circle (you are at the center)– Location: halfway between Zenith and Nadir

• North Celestial Pole

Directions in the Sky

• Azimuth = direction (N, S, SSW, etc) you face– Measured in degrees along horizon turning eastward from N

– Examples: 45 º azimuth = NE; 90º = E; 270º = W

• Altitude = how high in sky– Measured in degrees above horizon

Motion of Objects in the Sky

• What do you know about the motion of:– the Sun?– the Stars?

You observe a star rising due east. When this star reaches its highest position above the horizon, where will it be?

a) high in the northern sky

b) high in the eastern sky

c) high in the southern sky

d) high in the western sky

e) directly overhead

c) high in the southern sky

In-class exercise #1 - “Position”Pages 1-2 in workbook

1) How much of the celestial sphere can an Earth observer see at one time?

a) less than halfb) exactly halfc) more than half

b) exactly half

The Spinning Celestial Sphere

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Looking North Looking South

Why do the stars move?

• The Earth Rotates (from W to E)– appears to us as if the sky (the Celestial Sphere)

rotates (from E to W)

• Path of Stars– Stars “attached” to celestial sphere– Path is a circle (like latitude circle)– Called diurnal circle (diurnal = daily)

Earth inside Celestial Sphere

Locations on Celestial Sphere

• Projection of Earth’s rotation axis– North Celestial Pole / South Celestial Pole

• Projection of Earth’s Equator– Celestial Equator– Shape: circle– Location: halfway between the Celestial Poles

Rise / Set / Transit• Rise - move above horizon (appear)

• Set - move below horizon (disappear)

• Objects rise “in east” and set “in west”

• Transit - moving past highest point in path

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E S W

Circumpolar

• Some stars never rise or set– These stars are circumpolar

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Rotation at Different Latitudes

•Altitude of Pole = Latitude of Observer•Circumpolar zone depends on latitude

Imagine you are standing at the North Pole. Of the stars that you can see, roughly howmany of these stars are circumpolar?

a) Noneb) less than halfc) more than halfd) all

d) all

Imagine you are standing on the Equator. Of the stars that you can see, roughly how many of these stars are circumpolar?

a) Noneb) less than halfc) more than halfd) all

a) None

In-class exercise #2 - “Motion”Pages 3-6 in workbook

1. You are looking toward the north and see the Big Dipper to the right of Polaris. Fifteen minutes later, the Big Dipper will appear to have moved in roughly what direction?

a) east (to your right)b) west (to your left)c) up (away from the horizon)d) down (closer to the horizon)

?

2. At what time will star B appear highest in the sky?

a) early in the morningb) around noonc) in the afternoond) in the eveninge) around midnight

Midnight

6am

Noon

3. When star A is just above the eastern horizon, in what direction is star A moving?

a) up and to the northb) westc) up and to the southd) south

4) You are observing the sky from your southern hemisphere location in Australia. You see a star rising directly to the east. When this star reaches its highest position above the horizon, where will it be?

a) high in the northern skyb) high in the eastern skyc) high in the southern skyd) high in the western skye) directly overhead

a) high in the northern sky

Motion of the Sun• Diurnal (daily) motion like stars

– Sunrise “in east”– Transits “high” in south = Noon – Sunset “in west”– Altitude at noon depends on time of year

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Time of Day• Meridian:

– circle halfway between east and west

– Stars, etc. are highest when they Transit the meridian

• Time of day = solar position w.r.t transit (Noon)– am = ante meridian

– pm = post meridian

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11 am

10 am 2 pm

1 pm

Meridian

S

(6am) E (6pm)W

Motion of the Sun

• Annual (yearly) motion– Earth orbits Sun once per year– Sun seen in front of different constellations

throughout year

Annual Motion of Sun

Motion of the Sun

• Annual (yearly) motion– From day to day, Sun “slips” a little bit on

Celestial Sphere– Appears to shift all the way around the Celestial

Sphere once per year– Appears to move “from W to E” relative to the

background of stars– So from day to day, any given star rises earlier

SOHO Observes Solar Motion

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Annual Path of Sun

• Ecliptic – path of sun around celestial sphere– shape: circle

• Zodiac– Set of 12 Constellations containing Ecliptic– Sun in each constellation for about one month

• (solar) signs of the zodiac

For today, you should have done:

Homework: “Seasonal Stars”Exercise in workbook (p. 7-10)

Let’s go over the workbook exercise…

One evening at midnight, you observe Leo high in the southern sky at midnight. Virgo is to the east of Leo and Cancer is to the west. One month earlier, which of these constellations was high in the southern sky in at midnight?

a) Leob) Virgoc) Cancer

You go out tonight and see the brightest star in the constellation Orion just rising above your eastern horizon at 10 PM. One week later at 10 PM this same star will be

a) slightly higher in the sky.b) at the same height as before.c) below your horizon.d) setting on your western horizon.

One night, you see the star Sirius rise at exactly 7:36pm. The following night it will rise

a) slightly earlier.b) at the same time.c) slightly later.

The Day

• 1 day = time for object to return to same point on sky (e.g. transit to transit)

• Solar (Sun) day– Time from noon until next noon

• Sidereal (star) day– Time for a star to return to same point.

• Solar day is ~4 min longer than Sidereal day

In-class exercise“Solar vs. Sidereal Day”Pages 11-12 in workbook

What component of Earth’s motion causes the stars to rise earlier on successive nights?

a) its rotation about its axisb) its orbit around the Sunc) the tilt of its rotation axis

Which takes longer to complete?a) one solar dayb) one sidereal dayc) Both take the same amount of time.

Solar Day = 24 hours Sidereal Day = 23 hrs 56 min

Ecliptic on Celestial Sphere

•Earth’s axis tilted 23º with respect to orbit

Locations on Ecliptic

• Solstice: sun stops (moving N or S)– Summer Solstice

• Jun 21=1st day of summer

• Sun farthest N (from celestial equator)

• Longest day of year

– Winter Solstice• Dec 21=1st day of winter

• Sun farthest S (from celestial equator)

• Shortest day of year

Locations on Ecliptic

• Equinox: equal night and day– Vernal Equinox

• ~March 21 = 1st day of spring• Sun on equator (crossing from S to N)

– Autumnal Equinox• ~Sept 21 = 1st day of fall (autumn)• Sun on equator (crossing from N to S)

– Equinoxes are intersection points of Ecliptic and Celestial Equator

Diurnal Path of Sun Revisited• Summer: Sun above Celestial Equator

– Sun high in south at noon– Days are long

• Winter: Sun below Celestial Equator– Sun low in south at noon– Days are short

Zenith

NS

E

W

For today, you should have done:

Homework: “Path of the Sun”Exercise in workbook (p. 19-21)

Let’s go over the workbook exercise…