The SkyThe Sky

Essential QuestionsEssential Questions

►How do astronomers refer to How do astronomers refer to stars?stars?

►How can you compare the How can you compare the brightness of the stars?brightness of the stars?

►How does the sky move as How does the sky move as Earth rotates?Earth rotates?

►What causes the seasons?What causes the seasons?

ConstellationsConstellations

►ConstellationsConstellations are groups of are groups of stars that appear stars that appear relatively close relatively close together and together and which are said to which are said to represent represent people, gods, people, gods, and objects.and objects.

What we see…What we see…

AsterismsAsterisms

►Many groupings we recognize in the Many groupings we recognize in the sky are only parts of constellations sky are only parts of constellations or parts of multiple constellations. or parts of multiple constellations. Such groupings are called Such groupings are called asterismsasterisms..

►Examples include: Examples include: The Big DipperThe Big Dipper in in Ursa Major, the Ursa Major, the Great Square of Great Square of PegasusPegasus in Pegasus and Andromeda, in Pegasus and Andromeda, and the and the Summer TriangleSummer Triangle in Lyra, in Lyra, Aquila, and Cygnus.Aquila, and Cygnus.

Stellar NomenclatureStellar Nomenclature(star naming)(star naming)

► Names of constellations are in Latin.Names of constellations are in Latin.► Many bright stars also have names and Many bright stars also have names and

most names derive from Arabic. For most names derive from Arabic. For example: Betelgeuse, the bright red star example: Betelgeuse, the bright red star in Orion, comes from the Arabic in Orion, comes from the Arabic yad al-yad al-jawzajawza, meaning “armpit of Jawza , meaning “armpit of Jawza [Orion].” [Orion].”

► We’ll learn some more star names later, We’ll learn some more star names later, but astronomers have another way to but astronomers have another way to identify stars…identify stars…

Start with brightest and go Start with brightest and go GreekGreek

►Another way to identify stars is to Another way to identify stars is to assign Greek letters to the bright assign Greek letters to the bright stars in a constellation in stars in a constellation in approximate order of brightness.approximate order of brightness.

►The brightest star would be The brightest star would be designated by alpha (designated by alpha (αα) and the ) and the next by beta (next by beta (ββ), then gamma (), then gamma (γγ), ), delta (delta (δδ), etc.), etc.

►Now an example:Now an example:

Nomenclature ExampleNomenclature Example

►The brightest star in the The brightest star in the constellation Canis Major (The Great constellation Canis Major (The Great Dog) would be called Dog) would be called

►αα Canis Majoris Canis Majoris►This star happens to be the This star happens to be the

brightest star in the night sky and is brightest star in the night sky and is more commonly known by its Arabic more commonly known by its Arabic name—name—

►SiriusSirius►That reminds me of something…That reminds me of something…

Brightness of StarsBrightness of Stars

► The brightness of The brightness of stars is measured stars is measured on the on the magnitude magnitude scalescale. .

► Invented by Greek Invented by Greek astronomer astronomer Hipparchus.Hipparchus.

► Ancients divided Ancients divided stars in six stars in six divisions.divisions.

► The brightest stars The brightest stars were of 1were of 1stst magnitude.magnitude.

► The dimmest stars The dimmest stars were of 6were of 6thth magnitude.magnitude.

► Modern astronomers Modern astronomers have extended the have extended the scale to include scale to include fainter and brighter fainter and brighter objects.objects.

Magnitude ScaleMagnitude Scale► Bright objects Bright objects

have a small or have a small or negative negative magnitude.magnitude.

► Dim objects have a Dim objects have a large and positive large and positive magnitude.magnitude.

► Kind of backwards Kind of backwards isn’t it?isn’t it?

► 2.5 times for each 2.5 times for each differencedifference

► Apparent Visual Apparent Visual MagnitudeMagnitude

A Model of the SkyA Model of the Sky

►Ancient astronomers believed the sky Ancient astronomers believed the sky was a great sphere surrounding Earth was a great sphere surrounding Earth with the stars stuck on the inside like with the stars stuck on the inside like thumbtacks in a ceiling.thumbtacks in a ceiling.

►This This Celestial SphereCelestial Sphere model is not model is not true, but can you see why the true, but can you see why the ancients believed it? Have you ancients believed it? Have you observed the motions of objects in the observed the motions of objects in the sky?sky?

Celestial SphereCelestial Sphere

►At the At the north north celestial polecelestial pole is a is a star…star…

►Polaris (North Star) Polaris (North Star) in the constellation in the constellation Ursa Minor (Little Ursa Minor (Little Dipper)Dipper)

►There is no bright There is no bright star directly at the star directly at the south celestial polesouth celestial pole..

Some definitions:Some definitions:

►ZenithZenith—the point on the sky directly —the point on the sky directly above your head.above your head.

►HorizonHorizon—the circle on the sky where it —the circle on the sky where it meets the ground.meets the ground.

►Celestial EquatorCelestial Equator—the circle on the —the circle on the “celestial sphere” halfway between “celestial sphere” halfway between north and south celestial poles.north and south celestial poles.

►EclipticEcliptic—the path of the Sun in the sky.—the path of the Sun in the sky.

Circumpolar Circumpolar ConstellationsConstellations

► Circumpolar Circumpolar constellationsconstellations are are those that circle those that circle the pole star, the pole star, Polaris. Polaris.

► They are always They are always visible from Ohio. visible from Ohio. Just look north!Just look north!

► Constellations in Constellations in the south are the south are different…more on different…more on that later.that later.

PrecessionPrecession

► The Earth spins like a The Earth spins like a spinning top tipped at spinning top tipped at a 23.5a 23.5° angle.° angle.

► As tops do As tops do sometimes, the Earth sometimes, the Earth wobbles and its north wobbles and its north pole points at pole points at different places. This different places. This is called is called precessionprecession..

► An entire wobble An entire wobble takes about 26,000 takes about 26,000 years!years!

The ZodiacThe Zodiac

Zodiacal ConstellationsZodiacal Constellations►Do you see why the zodiacal Do you see why the zodiacal

constellations are only visible constellations are only visible during certain seasons?during certain seasons?

►I was born in June and my I was born in June and my “zodiacal sign” is Gemini. “zodiacal sign” is Gemini.

►When is the constellation When is the constellation Gemini visible in the sky?Gemini visible in the sky?

►What did that mean about the What did that mean about the sign under which I was born? sign under which I was born? Where was Gemini?Where was Gemini?

The SeasonsThe Seasons►Many people believe we have cool Many people believe we have cool

winters and warm summers winters and warm summers because the Earth gets closer and because the Earth gets closer and farther away from the Sun over farther away from the Sun over the year.the year.

►While the Earth’s orbit While the Earth’s orbit isis elliptical, elliptical, and we do get about 1.7% and we do get about 1.7% closer/farther to the Sun than closer/farther to the Sun than average, this is average, this is notnot the cause of the cause of the seasons.the seasons.

Perihelion & AphelionPerihelion & Aphelion►In fact, the Earth is at In fact, the Earth is at perihelionperihelion, ,

its closest point to the Sun, its closest point to the Sun, around January 4around January 4thth when we have when we have winter. (~147,500,000 km)winter. (~147,500,000 km)

►The Earth is at The Earth is at aphelionaphelion, its , its farthest point from the Sun, farthest point from the Sun, around July 4around July 4thth during our during our summer. (~152,500,000km)summer. (~152,500,000km)

What causes the seasons What causes the seasons then?then?

► The tilt of the The tilt of the Earth’s axis.Earth’s axis.

► Throughout the Throughout the year, the Earth’s year, the Earth’s North Pole North Pole always points always points towards Polaris.towards Polaris.

► Sunlight hits the Sunlight hits the Earth at different Earth at different angles in the angles in the four seasons.four seasons.

The SeasonsThe Seasons►In the summer, the Sun is above In the summer, the Sun is above

the horizon longer because it the horizon longer because it rises north of east and sets rises north of east and sets north of west. (see diagram)north of west. (see diagram)

►It also shines more directly It also shines more directly down on the ground. (think of down on the ground. (think of flashlight shining straight down flashlight shining straight down or at an angle)or at an angle)

►Both effects cause warmer Both effects cause warmer weather.weather.

SolsticesSolstices►The Sun is highest in the sky on The Sun is highest in the sky on

the the Summer Solstice,Summer Solstice, June 21 June 21stst or or so. On this day, the Sun rises so. On this day, the Sun rises the farthest N of E and there are the farthest N of E and there are the most daylight hours.the most daylight hours.

►The Sun is lowest in the sky on The Sun is lowest in the sky on the the Winter SolsticeWinter Solstice, December , December 2121stst or so. On this day, the Sun or so. On this day, the Sun rises the farthest S of E and rises the farthest S of E and there are the least daylight there are the least daylight hours.hours.

EquinoxesEquinoxes►On March 21On March 21stst or so, the or so, the Vernal Vernal

(Spring) Equinox(Spring) Equinox occurs. The occurs. The Sun rises due East and we Sun rises due East and we have equal hours of daylight have equal hours of daylight and night.and night.

►On September 21On September 21stst or so, the or so, the Autumnal (Fall) EquinoxAutumnal (Fall) Equinox occurs. The Sun again rises occurs. The Sun again rises due East and we have equal due East and we have equal hours of day and night.hours of day and night.

Direct Sunlight on Solstices Direct Sunlight on Solstices and Equinoxesand Equinoxes

►On the Equinoxes, the Sun’s rays On the Equinoxes, the Sun’s rays shine straight down on the shine straight down on the Earth’s Equator.Earth’s Equator.

►On the Summer Solstice, the Sun On the Summer Solstice, the Sun shines straight down on the shines straight down on the ‘Tropic of Cancer’ (23.5° North).‘Tropic of Cancer’ (23.5° North).

►On the Winter Solstice, the Sun On the Winter Solstice, the Sun shines straight down on the shines straight down on the ‘Tropic of Capricorn’ (23.5° ‘Tropic of Capricorn’ (23.5° South).South).