Physics Department 9/9/2018

Topics for Today’s Class – Ch2 PHYS 1403 and Galaxies 1. The Celestial a) Important Markers b) ’s axis of c) Local d) Precession 2. Coordinate System in a) Horizon Coordinate System b) Equatorial Coordinate System

The and Celestial Motions The Celestial Sphere (CS)

• The sky above seems like a great blue dome in the and a sparkling ceiling at night • The Early represented the sky as a sphere and called it celestial sphere • Lets study the celestial sphere (CS)

The Celestial Sphere The Celestial Sphere (CS) • From geographic latitude l (northern hemisphere), you see the celestial north pole l degrees above the horizon; • • From geographic latitude –l (southern hemisphere), you see the celestial – on the celestial sphere (CS) directly south pole l overhead degrees above 90o - l • Nadir the horizon. • Celestial – Point on the CS directly underneath l culminates • 90º – l above – Projection of Earth’s equator onto CS the horizon. • North – Projection of Earth’s north pole onto CS

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Where is your Celestial Equator in the Sky? How to Find your Latitude?

• Look North in the night sky and find Celestial Celestial Polaris. Measure the from horizon North Pole Equator using your fist (ballpark) or • Use a world map (most accurate) 40.7º 49.30 Horizon • Google it (quick and easy) North South

New York City: l ≈ 40.7º

Why Knowing Latitude is Important? The • Travel from north pole to south pole . North pole: no southern visible . As you travel south you latitude goes from +90 to 0 and -90 you see more and more southern and less of norther constellation . At equator you see most of northern and southern constellations. . Your Latitude determines the marker for celestial , and also long ago a very important tool for sailors on ships for navigation on world oceans.

The Sun’s apparent path on the sky is called the Ecliptic. Equivalent: The Ecliptic is the projection of Earth’s orbit onto the celestial sphere.

Wikipedia:CC BY-SA 3.0

The Ecliptic, Celestial Equator and Precession At left, gravity is pulling on a slanted top. => Wobbling the Horizon on Celestial Sphere around the vertical.

The Sun’s gravity is doing the same to Earth. The resulting “wobbling” of Earth’s axis of rotation around the vertical w.r.t. the Ecliptic takes about 26,000 years and is called precession.

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Precession Precession Animation As a result of precession, the celestial north pole follows a circular pattern on the sky, once every 26,000 years.

It will be closest to Polaris ~ A.D. 2100. There is nothing peculiar about Polaris at all (neither particularly bright nor nearby etc.) ~ 12,000 years from now, it will be close to in the constellation Lyra. https://www.youtube.com/watch?v=qlVgEoZDjok

How do I find a Celestial Object with How do you find any location on a World on my telescope? a map?

amy.com Wikipedia:CC BY-SA 3.0

How do you find any location on a World on a map? Celestial Coordinate Systems

• use coordinate systems just like the one we use to describe the positon of cities on a map. • We will study two different commonly use systems. Each have their advantage.

Wikimedia common:CC BY-SA lpc1.clpccd.cc.ca.us 3.0

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Horizon Coordinate System Equatorial Coordinate System • (δ or • Altitude (Alt) and (Az) Dec.) are like • Depends on the Observers horizon • Declination is the angle between the object and the celestial equator • It is measured in degrees from the Celestial equator. • 0 to +90 for North and 0 to -90 for South of Celestial equator Source: www.nmm.ac.uk/tserver.php?f=altaz-star2.jpg... Wikipedia:CC BY-SA 3.0

Equatorial Coordinate System What is a Vernal ?

• (α) • Vernal equinox is are like the intersection of • It is measured in the celestial hours, minute and equator and the seconds like a ecliptic circle clock time • On March 21st and nd • It is the angle September 22 between the object around noon the and the Vernal Sun is on these Equinox. intersection points • These points are • Vernal Equinox is called vernal and the ( autumnal zero point or RA = (equal 0). Wikipedia:CC BY-SA 3.0 day and night).

How to find where the vernal equinox is on any date and What does Right Ascension Angle mean? time?

1.Suppose the RA of an object A is 3 hours 40 minutes 10 1.We know that the celestial equator culminates at 90- seconds When it is overhead (Zenith) in the sky. latitude from our south (90-32 = 58 degrees for 2.One hour later this object will be further west and still its Stephenville). right ascension would be the same but now an object B 2.The angle between the two equinox is 180 degrees so with RA of 4h 40m 10s would be at Zenith. 180 degrees = six months or 3.The great circle that passes at RA 0h is the vernal equinox 1 month = 30 degrees or and this means that your object A and B are east of the 1 day = 1 degree (base on 30 day month for approximate vernal equinox. calculations) 3.So if we compute how many days and hours have elapsed from the last equinox, we can calculate approximately where the vernal equinox is on any date and time. 4.Computers can do these very precisely.

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Acknowledgment

• The slides in this lecture is for Tarleton: PHYS1411/PHYS1403 class use only • Images and text material have been borrowed from various sources with appropriate citations in the slides, including PowerPoint slides from Seeds/Backman text that has been adopted for class.

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