The Starry Sky

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There are roughly 6000 or so stars visible to the naked eye

Orion

: Are these stars physically related?

Green

= Yes,

Purple

= No

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The stars are so distant that they appear only as points of light.

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We can detect color and brightness with the naked eye – but not much more.

The Ancient Observatory of Stonehenge

Unique Construction

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Pretend

the stars are painted on a giant ‘bubble’ around the Earth known as the celestial sphere Stars are at different distances The stars of a constellation only appear to be close to one another, they may be located at very different distances from us.

Constellations

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Stars are grouped into “easily” recognized shapes called constellations.

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There are 88 official constellations

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The sky is completely mapped by the constellations.

Early Sky Map Lots of art work

Today , constellations are well-defined regions on the sky , irrespective of the presence or absence of bright stars in those regions.

Some sky maps leave out the boundaries ; size of star depends upon magnitude (how bright).

Constellations: Three interpretations of the Winter Constellation Orion:

Early The Sky Modern Maps

Star Names an example Bayer System Name

     

alpha Orionis Betelgeuse beta Orionis Rigel delta Orionis Mintaka gamma Orionis Bellatrix epsilon Orionis Alnilan kappa Orionis Saiph Bayers’ System Stars are named by a G reek letter (

, , 

) according to their relative brightness within a given constellation + the possessive form of the name of the constellation. Catalogues HD, ASO

Sagittarius the archer ( better known as the teapot an asterism)

Stars can be used to locate other stars and constellations. Star hopping

To find your directions in the sky, find the north star, put your back to the north star and you will be looking southward.

East will be on your left, with west on your right.

Facing due south you are you are looking along your meridian.

Celestial Motions

Diurnal (Daily) Motions

• •

The Sun , Moon , Stars and Planets rise in east, set in the west. These motions are all due to Earth’s Rotation about Its Own Axis (1 Day = 24 hours (almost).

The Moon, and Planets have their own motion, which is from west to east moving counter clockwise around the Sun..

Annual (Yearly) Motions

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Position of Stars, Constellations

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They rise 4 minutes earlier each night.

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Different constellations are visible at different times of the year .

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Position of the Sun

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The position of Sun on horizon shifts back and forth and the peak height in the sky changes.

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Seasons

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Length of a day, night, and the climate changes over course of the year.

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Earth orbits Around the Sun, one year = 365 Days (almost)

Other Changes Phases of the Moon (monthly)

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Moon rises about 50 minutes later each night. Takes 29.5 days from New Moon to the next New Moon.

Change in “Celestial Poles ”

The “North Star ( Polaris)” has not always been the North Star.

Eclipses Sun or Moon becomes “blocked” from view during a year.

The Horizontal System

Zenith

Local Sky

W S

Horizon

E

Zenith is a point on the celestial sphere directly above your head.

N

Celestial Meridian

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A line from north to south, through your Zenith is called the Celestial Meridian.

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When objects are on the Meridian at their highest point in the sky, we say they are in transit at this point.

.

Astronomical noon is when Sun is on the Meridian, not necessarily straight up.

North Pole

Meridian Equator

South Pole

Altitude is measured from your horizon up to the celestial object in degrees.

Zenith

Celestial Sphere

Meridian Altitude East North South West Nadir

Azimuth is measured along the horizon from a point below the North Pole, which is zero degrees, to a point directly below the object on the horizon.

Zenith

Celestial Sphere

Meridian Altitude East North South West Azimuth Nadir

North is 0 deg, East is 90 deg, South is 180 deg and west is 270

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For a geographic latitude of L degrees on the northern hemisphere , you will see the celestial north pole L degrees above the horizon.

Zenith

Your Latitude = altitude of celestial pole

NCP CEq

S E W

Horizon Your Latitude

N

The Equatorial System

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Celestial Co-ordinates

The altitude and azimuth of a star change all the time, and depend on where you are.

We need co-ordinates “fixed” to the celestial sphere.

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Declination and Right Ascension

A star’s Dec & RA will change very little in our lifetime.

It seems as though there is a sphere of stars surrounding the earth.The Celestial Sphere is an extension of the Earth.

North Pole

Celestial Sphere Celestial Equator

South Pole South Pole Earth

Earth

Places on the Earth are located by Latitude & Longitude

Declination (Dec) is analogous to latitude and is measured in degrees up and down from the celestial equator. (Max is +90 deg or –90 deg)

North Celestial Pole (NCP) Declination (Dec) Celestial Equator

Right Ascension is analogous to longitude, but is measure in time units of hours, min and sec, from the zero point located at the vernal equinox. (0 hours to 24 hours)

North Celestial Pole (NCP)

15 deg = 1 hr

Celestial Equator (zero of Dec) Right Ascension (RA) (zero of RA)

Right Ascension & Declination With these two directions, any object can be located on the celestial sphere.

North Celestial Pole (NCP) Celestial Equator (zero of Dec) Right Ascension (RA) Declination (Dec) (zero of RA)

Ecliptic vs. Celestial Equator

The Earth’s axis of rotation is inclined to its orbital plane by 23.5

0

With no tilt, the planets would all seem to be located somewhere on the Celestial Equator, straight out from the Earth’s equator.

The planets are revolving around the Sun, but they would appear to be located along the equator.

Celestial Equator The Sun does not move, the Earth’s motion makes the Sun appear to move.

Because the Earth is tilted 23 1/2 degrees, the Sun & Planets move in a different location, as seen from Earth.

The Ecliptic is the path of the Sun, Moon & Planets revolving around the Sun as seen from Earth.

0 RA where the Sun on the Ecliptic crosses the CE on its way upward

The position of Sun on horizon shifts back and forth, so that the peak height in sky changes

• • • •

Finding North The big dipper does not set from most of the USA.

The two end stars point to Polaris which happens to be near the celestial pole The big dipper acts like a big clock: It goes once round in 1 a day

Star Trails at our location



Looking north you’ll see the stars circle around Polaris.



Looking at stars rising roughly east you’ll see them rise upward, go to highest point then start downward setting in the west.



Looking toward the south you’ll see stars rise in the south east, go to the highest point and set in the southwest, a shorter arc across the sky.

What you see depends on your latitude At the North Pole

NCP Z North Pole

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If you were at the north pole of the Earth, Polaris would be overhead. All stars would go round the north celestial pole never setting: all are circumpolar

At the Equator

Z S E Equator W N NCP

Stars that never set , and go round and round the pole are called circumpolar stars.

90 deg – your latitude is the Declination of the limit of circumpolar stars. For Atlanta, 90 deg- 33 deg 45 min = 56 deg 15 min. So stars southward of this point are not circumpolar.

The Seasons

Inclined Pole Causes Seasons Earth’s equator is inclined against the ecliptic by 23.5

º .

Remember the Sun does not move, the Earth moves.

The change in position of the rising sun.

June (Summer Solstice) March & September (Equinoxes) December (Winter Solstice) 0 23.5

-23.5

1 KW/m 2 1 m 2 2 m 2 1 KW/m 2

The Tropic of Cancer & Capricorn

Measuring angles

Angular distance Basic unit is the degree 1° 1° split into 60’ minutes of arc 1’ split into 60’’ seconds of arc 1° is 3600” seconds of arc

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The easiest way to estimate relative angular distances on the sky without any equipment is to use your outstretched hand like an angular ‘ruler’ Outstretched Hand as an Angular Ruler

The Earth and its Motions

Earth’s rotation is causing the day/night cycle.

The Suns motion, which is really the movement of the Earth around the Sun, causes the Sun to appear to move through the same constellations at certain times of the year.

There are actually 13 constellations along the ecliptic CONSTELLATION TRADITIONAL ACTUAL (2000) Capricornus Dec. 22 - Jan. 21 Jan. 21 - Feb. 16 Aquarius Pisces Jan. 22 - Feb 21 Feb. 16 - Mar. 11 Feb. 22 - Mar. 21 Mar. 11 - Apr. 18 Aries Taurus Gemini Cancer Mar. 22 - Apr. 21 Apr. 18 - May 13 Apr. 22 - May 21 May 13 - Jun. 22 May 22 - Jun. 21 Jun. 22 - Jul. 21 Jun. 22 - Jul. 21 Jul. 21 - Aug. 10 Leo Virgo Libra Scorpius Ophiuchus Sagittarius Jul. 22 - Aug. 21 Aug. 22 - Sep. 21 Sep. 22 - Oct. 21 Oct. 22 - Nov. 21 Nov. 22 - Dec. 21 Aug. 10 - Sep. 16 Sep. 16 - Oct. 31 Oct. 31 - Nov. 23 Nov. 23 - Nov. 29 Nov. 29 - Dec. 18 Dec. 18 - Jan. 21 DAYS 26 24 38 25 40 29 20 37 45 23 6 19 34

Precession

Gravity is pulling on a slanted top Wobbling around the vertical. The Sun’s gravity is doing the same to the Earth.

The resulting “wobbling” of the Earth’s axis of rotation around the vertical takes about 26,000 years, and is called precession

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Except for presently being the pole star, there is nothing special about Polaris.

Because of precession, the celestial north pole follows a circular pattern 26,000 years .

on the sky, once every It will be closest to Polaris ~ A.D. 2100 . 12,000 years from now , it will be close to Vega in the constellation Lyra.

Messier Catalogue

• Charles Messier compiled a list of approximately 100 diffuse objects that were difficult to distinguish from comets.

Star Clusters Nebula Galaxies

Deep Sky Object Catalogs

Messier Catalog

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e.g. M1, M2, M3, etc.

New General Catalog

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e.g. NGC 7293 Index Catalog

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- e.g. IC302

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Solar day (noon-to-noon = 24 hours) is rotation period with respect to the Sun



4 minutes longer than the true rotation period of the Earth with respect to the stars called the sidereal day.

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Roman Julian Calendar year

=

365 d + leap yr

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Problem with Julian Calendar



average yr is 365.25 d, 11 minutes too long !

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By the 1500’s, the time of Pope Gregory, the calendar was ahead of astronomical time keeping by more than two weeks Gregorian-Julian Reform: Calendar Century years not divisible by 400 are ordinary years, not leap years ( Example: 1700 AD was not a leap year, but 2000 AD was)

SFA Star Chart 1

Thanks to the following for allowing me to use information from their web site : Nick Stobel Bill Keel Richard Pogge John Pratt NASA, JPL, OSHO