Goal: To understand how objects move in the night sky over any time span, and what we can discover from that. Objectives: 1)To be familiar with Basic Sky terms. 2)To learn about movements in the sky during 1 night. 3)To learn about movements in the sky during the year. 4)To learn about movements in the sky over thousands of years. 5)To understand what we can use these movements for.

Basic Sky Terms ?01http:// ?01 Celestial Sphere – if all night sky objects where on a hollowed sphere, this sphere is the Celestial Sphere. Celestial N. and S. Poles, and Equator – expansions of the Poles and Equator of the earth. On Celestial Sphere – position of Stars are constant! The sun, moon, and planets all move along this sphere.

Basic Sky Terms continued Angel of Declination – think of this as the object’s Latitude on the Celestial Sphere. Angle of Right Ascension – think of this as the object’s Longitude on the Celestial Sphere. Ecliptic Plane – the plane which the sun and the planets trace as they move across the Celestial Sphere.

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Motions over 1 night. Most objects in the night sky (with the exception of the moon) move 15 degrees per hour in a circle around the null point. Image courtesy of Astronomy Picture of the Day 12/1/1996

Motions over the course of a year Stars stay fixed to the Celestial Sphere The moon moves around the Sphere once per month (we will cover this more later). The planets “wander” through the Celestial Sphere also, along the Ecliptic Plane. – Most of the time they move from right to left through the Celestial Sphere (also known as “prograde”) along the. However, sometimes they turn and move in the “retrograde” direction.

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Motion over 1 year, the sun and stars. The sun every year moves 1 full circle along the Ecliptic Plane. (image below courtesy of

Side effect of the sun’s movement: At different times of the year you see different stars. (image below courtesy of

During the time of Ancient Greece, what object was at the North Celestial Pole? 1)Nothing 2)Vega 3)The Moon 4)Polaris (aka The North Star)

Motions over thousands of years: Stars will move some due to “proper motion”. “Proper motion” is just a fancy way of saying that the sun and the stars all move, but that they are so far apart that it takes a long time for this motion to be observable. The more important motion here is that of precession. The direction of the tilt of the earth “precesses” or rotates once every 26,000 years. So, every 2,000 years, the time of month that each constellation is viewable changes by 1 month.

What can we use all these movements for?

1)Growing crops (from the yearly motion). 2)Telling time. 3)Navigate (astrolabe). 4)Calendar. 5)Find North or South.

Conclusion We have learned that stars have a mostly fixed place on the Celestial Sphere, such that what you can see and when you see it is determined by the rotation of the earth, and the position of the sun in the sky. Planets are somehow special. We can use these movements for many different purposes, which ancient civilizations did!