# Measuring the Sky - Ann Arbor Earth Science

Measuring the Sky - Ann Arbor Earth Science

1,000 10,000 kilometers (106 (107 m)m)•© 2004 Pearson Education Inc., publishing as

1,000 10,000 kilometers (106 (107 m)m)•© 2004 Pearson Education Inc., publishing as

10,000 kilometers (104(107m)m)

10,000 100 meters kilometers (102 m) (107m)

How old is theUniverse?•The CosmicCalendar– if the entire ageof the Universewere onecalendar year– one monthwould beapproximately1 billion realyears

The Celestial Sphere•The sky above looks like adome…a hemisphere..•If we imagine the skyaround the entire Earth, wehave the celestial sphere.•This a 2-dimensionalrepresentation of the sky¥Because it represents ourview from Earth, we placethe Earth in the center ofthis sphere.

The Celestial SphereNorth & South celestial polesthe points in the sky directly above the Earth’sNorth and South polescelestial equatorthe extension of the Earth’s equator onto thecelestial sphereeclipticthe annual path of the Sun through the celestial sphere,which is a projection of ecliptic plane

•© 2004 Pearson Education Inc., publishing as

A spinningimaginaryCelestialSpheresurroundingEarth aids innavigatingthe sky

Measuring the SkyWe measure the sky in angles, not distances.•Full circle = 360º•1º = 60 arcmin•1 arcmin = 60 arcsec

Angular Measurements and Notation:•Full circle = 360º•1º = 60 (arcminutes)•1 = 60 (arcseconds)

What is 55.435 degrees in degreesminutes-secondsnotation?55deg0.435(60) 26.1arcmin26 arcmin0.1(60) 6arcsecso,55o26'06"What is73 o 45 33.56in decimal degrees?73deg45/60 .7533.56/360073.7593220.009322

Problem•45.635 degrees is how many degrees, arcminutes, and arcseconds?•How many degrees, arcminutes, and arcseconds does the moon moveacross the sky in one hour? (the lunar day is 24 hours and 48 minuteslong)•The moons diameter is about 30 arcminutes, so find out how long ittakes for the moon to travel its diameter.

Answer•45.635 degrees is how many degrees, arcminutes, andarcseconds?45deg0.635(60) 38.1arcmin38 arcmin0.1(60) 6arcsec•How many degrees, arcminutes, and arcseconds does the moonmove across 24 hoursthe 48 minutes sky one 24.8hour? hours (the lunar day is 24 hours and48 minutes long)The moon moves360 degrees during360deg24.8hours14.516129deg/ hour14that time so :o30 58.06"•The moons diameter is about 30 arcminutes, so find out how long0.5degit takes vfor d/t the moon t d/v to travel its diameter. 0.034 hour14.516129deg/ hour(0.034hour)(60minutes/hour) 2.1minutes 2 6

Review: Coordinates on the Earth•Latitude: position north or south of equator•Longitude: position east or west of primemeridian (runs through Greenwich,England)

Review: Coordinates on the Earth•Latitude: position north or south of equator•Longitude: position east or west of primemeridian (runs through Greenwich,England)

Review: Coordinates on the Earth•Latitude: position north or south of equator•Longitude: position east or west of primemeridian (runs through Greenwich,England)

© 2004 Pearson Education Inc., publishing as

A spinningimaginaryCelestialSpheresurroundingEarth aids innavigatingthe sky

Measuring Angles in the Sky

The Local Skyzeniththe point directly above youhorizonall points 90° from the zenithaltitudethe angle above the horizonmeridiandue north horizon zenith due south horizon

To pinpoint a spot in the local sky:Specify altitude anddirection along the horizon

Elements of the equatorial coordinatesystem on the celestial sphere•Vernal Equinox: The position ofthe Sun on the first day ofspring (Sets the prime meridian)•Right Ascension: How far eastof the Vernal Equinox an objectis located (longitude)•Celestial Equator: The lineseparating the celestial sphereinto northern and southernhalves.•Declination: How far above orbelow the celestial equator anobject is located.(latitude)

The Daily Motion•As the Earth rotates, the skyappears to us to rotate in theopposite direction.•The sky appears to rotate aroundthe N (or S) celestial poles.•If you are standing at the poles,nothing rises or sets.•If you are standing at the equator,everything rises & sets 90 tothe horizon.

The Daily Motion•As the Earth rotates, the skyappears to us to rotate in theopposite direction.•The sky appears to rotate aroundthe N (or S) celestial poles.•If you are standing at the poles,nothing rises or sets.•If you are standing at the equator,everything rises & sets 90 tothe horizon.

The Daily Motion•As the Earth rotates, the skyappears to us to rotate in theopposite direction.•The sky appears to rotate aroundthe N (or S) celestial poles.•If you are standing at the poles,nothing rises or sets.•If you are standing at the equator,everything rises & sets 90 tothe horizon.

Time Exposure Photograph:• Estimate the exposure time• Which direction did stars move?

Annual Motion•As the Earth orbits the Sun, the Sun appears to moveeastward with respect to the stars.•The Sun circles the celestial sphere once every year.

Annual Motion•As the Earth orbits the Sun, the Sun appears to moveeastward with respect to the stars.•The Sun circles the celestial sphere once every year.

Annual Motion•As the Earth orbits the Sun, the Sun appears to moveeastward with respect to the stars.•The Sun circles the celestial sphere once every year.

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