Summer Constellations

Dr. Christopher SirolaDepartment of Physics & Astronomy

University of Southern Mississippi

Contents• Introduction to Constellations

• Coordinate Systems• Locating Objects• A Few Stories• A Few Objects

Introduction to Constellations

The word “constellation” is a combination of “con” (“with”, as in grouping) and “stella” (star).

Modern constellations are official groupings of stars.

Constellations vs. Asterisms

Similarly, an “asterism” is also a picture made of stars. So what’s the difference?

An asterism is unofficial

(as determined by astronomers)

Constellations vs. Asterisms

The Big Dipper is an asterism; Ursa Major is a constellation.

Constellation Trivia• There are 88 official constellations

• There is no part of the sky not covered by modern constellations

• Most northern hemisphere constellation names taken from ancient Greeks• Many southern hemisphere

constellations named by 15th & 16th century European sailors

Celestial Coordinates

Recall coordinates used on Earth:• Longitude measures east-west• Latitude measures north-south• Poles & equator define system

The Sky is nicknamed the Celestial Sphere

On Earth In the SkyNorth Pole North Celestial PoleEquator Celestial EquatorSouth Pole South Celestial PoleLongitude Right AscensionLatitude DeclinationPosition of the Sun Ecliptic

NCP

SCP

Celestial Equator

Ecliptic (path of the Sun)

Earth

Declination (latitude)

Right Ascension (longitude)

Celestial CoordinatesRight Ascension

• Like longitude• East-west position• Divides full circle

into 24 “hour angles” rather than 3600

• Thus an object moves one hour per hour

• Goes from 0 to 24h

Declination• Like latitude• North-south position• Uses degrees

(like Earth latitude)• Goes from +900 (NCP)

through 00 (CE) to – 900 (SCP)

Sirius: RA 6 hours 45 minutes, Dec – 16 degrees

Sirius: RA 6 hours 45 minutes, Dec – 16 degrees

Can you locate some bright stars?

RA (h m) Dec (deg) Star Constellation 6 45 - 16 Sirius Canis Major 18 36 + 39 19 51 + 9 20 41 + 45 16 21 - 25 14 16 + 19

Can you locate some bright stars?

RA (h m) Dec (deg) Star Constellation 6 45 - 16 Sirius Canis Major 18 36 + 39 Vega Lyra 19 51 + 9 Altair Aquila 20 41 + 45 Deneb Cygnus 16 21 - 25 Antares Scorpius 14 16 + 19 Arcturus Boötes

Global vs. LocalThe Celestial Sphere is a global

system, used by everyone everywhere on Earth.

What do we use when we wish to find objects in one’s own sky?

Local CoordinatesTerm MeaningHorizon Divides Earth from skyZenith Point directly above one’s headMeridian Divides east from west on skyAltitude Angle of object above horizon

(from 0 to 90 degrees)Azimuth Angle of object around horizon

(measured east or west from due north, from 0 to 180 degrees)

Alt-Azimuth Coordinates

E

S N

W

zenith

horizon

meridianTo NCP

Alt-Azimuth Coordinates

E

S N

W

altitude

azimuth

StoriesWhy do constellations exist?

• Sometimes look like pictures• Help people find their way in the sky• Cosmological significance • Stories of heroes• Historical stories

Example: Lyra the Harp

Lyra is a harp, a stringed instrument held in one hand & plucked by the fingers of the other hand.

Why would it be in the sky?

Example: Lyra the Harp

In oral societies like ancient Greece, traveling musicians spread stories & shared culture.

A Greek musician would sing or recite poetry (like Homer’s Iliad) to music.

Lyra the Harp• Orpheus was the son of

Apollo and the muse Calliope• Orpheus was regarded as the

best musician ever• Orpheus was wedded to a

lady named Eurydice in an outdoors ceremony

• Eurydice was bitten by a snake & died from the poison

Lyra the Harp

Orpheus travels to the underworld to plead for Eurydice’s life before the throne of Hades

Lyra the Harp

Hades allows Eurydice to leave, provided Orpheus

doesn’t look for her until they

are completely back in the world

of the living.

Lyra the Harp

Orpheus looked back just before the exit; Eurydice vanished.

Lyra the Harp

• Years later, Orpheus falls afoul of a local king

• The king throws Orpheus into a pit of snakes without his harp

Lyra the Harp• Orpheus charms

the snakes with his voice alone…

• … except for the deaf snake…

• Orpheus’ harp had been thrown into a nearby stream

• The music leaked out as the babbling of brooks

Lyra the Harp

How can we locate Lyra in the night sky?

• Vega is very bright• Look for Vega near the zenith around

midnight during the summer• Vega is white in color

• Vega is near a parallelogram of medium-bright stars (forming the strings of the harp)

Vega: RA 18h 36m; Dec + 39 deg

The Summer Triangle: Vega, Deneb, Altair

Vega

Fun facts about Vega• 5th brightest star in night sky

• Original zero point for astronomical magnitude (brightness) system

• Normal (“Main Sequence”) star like the Sun, except a little bigger & brighter (Type A0 V)

• Was the North Star about 13,000 years ago (and will be so again in another 13,000 years!)

Other Summer Constellations• Cygnus (the swan)• Aquila (the eagle)• Hercules• Corona Borealis (the crown)• Bootes (the hunter)• Scorpius (the scorpion)• Sagittarius (the archer)• Ophiuchus (the doctor)

Would you like to hear about any of these?(if time allows)

Thanks, and clear skies!