1. Charting the Heavens

2. Standards Identify & apply measurement techniques
Understand the scale & contents of the universe
Predict changes in the positions and appearances of objects in the sky (e.g., moon, sun) based on knowledge of current positions and patterns of movement (e.g., lunar cycles, seasons)

3. Our Place in Space
We live on an ordinary rocky planet that orbits an average star, near the edge of a huge collection of stars, the Milky Way Galaxy, which is one galaxy among countless billions of others spread through the universe.

4. Our Place in Space
The universe is the totality of all space, time, matter and energy
Astronomy is the field of science that is concerned with the study of the universe

5. Our Place in Space
In order to study the universe, we have to consider things on scales that are unfamiliar from our everyday experience
For example: a common measure of distance is the light year – the distance traveled by light in one year
The speed of light (denoted by the letter c) is approximately 300,000 km/s or 186,000 mi/s
Therefore, the distance of a light year is 10 trillion km (6 trillion mi)

6. The Obvious View: Constellations in the Sky
We can see ~3000 stars from each hemisphere (6000 total) with the unaided eye.

7. The Obvious View: Constellations in the Sky
It is a natural human tendency to see patterns & relationships between objects even when no true connection exists

8. The Obvious View: Constellations in the Sky
Long ago people connected the brightest stars into configurations called constellations, which were named after mythological beings, heroes and animals
The stars that make up a constellation are NOT actually close to one another, and if viewed from a different location in space, they would not form the same pattern

9. Orion
Distances between
the stars of Orion

10. The Obvious View: Constellations in the Sky
Early astronomers studied the constellations for navigation, and as calendars to predict planting and harvesting seasons
In all, there are 88 constellations

12. The Obvious View: The Celestial Sphere
If you were to watch the sky over the course of a night, the constellations would seem to move from east to west.

13. The Obvious View: The Celestial Sphere
Ancient astronomers were aware that, though the stars moved across the sky, their relative positions to each other remained unchanged
They concluded that the stars must be firmly attached to a shell, or celestial sphere, surrounding Earth

14. The Obvious View: The Celestial Sphere
Today, we know the motion of the stars is due to the spin, or rotation, of the Earth on its axis
Even though we know the idea of a celestial sphere is incorrect, we use the idea as a convenient model to help us visualize the position of stars in the sky

15. The Obvious View: The Celestial Sphere
Celestial poles – the points where Earth’s axis intersects the celestial sphere
Celestial equator – midway between the north and south celestial poles, represents the intersection of Earth’s equatorial plane with the celestial sphere

16. Image: http://www. astro. columbia
sphere.jpg

17. The Obvious View: Celestial Coordinates
There are two methods of locating stars in the sky
The simplest method is to specify their constellation and then rank the stars in it in order of brightness
The brightest star is denoted by the Greek letter alpha (α), the second brightest by beta (β), etc.

18. The Obvious View: Celestial Coordinates
For more precise measurements, astronomers use a celestial coordinate system

19. The Obvious View: Celestial Coordinates
The latitude and longitude coordinate system that’s used on Earth is extended out to the celestial sphere
Declination (Dec) – the celestial equivalent of latitude.
It is measured in degrees north or south of the celestial equator.
The celestial equator has a declination of 0, the north celestial pole is +90, and the celestial south pole is –90

20. The Obvious View: Celestial Coordinates
Right ascension (RA) (the celestial equivalent of longitude) is measured in units called hours, minutes and seconds, and it increases in the eastward direction
Units of RA are constructed parallel to units of time

21. Image:
navy.mil/focus/space
sciences/images/
observingsky/celestial
sphere.jpg

22. Earth’s Orbital Motion: Day-to-Day Changes
We measure time by the sun
Solar day – the 24 hour day that is our basic unit of time. It is measured from one noon to the next
Diurnal motion – the daily progress of the sun & other stars across the sky

23. Earth’s Orbital Motion: Day-to-Day Changes
The stars’ positions in the sky do not exactly repeat from one night to the next
Each night, the whole celestial sphere appears to be shifted a little relative to the horizon

24. Earth’s Orbital Motion: Day-to-Day Changes
A day measured by the stars is called a sidereal day
It differs in length from a solar day
The difference between a solar day and a sidereal day is a result of Earth’s rotation on its axis along with its revolution about the sun
The solar day is 3.9 minutes longer than the sidereal day (which is 23h56m long)

25. Earth’s Orbital Motion: Seasonal Changes
The stars/constellations that we see in the sky are different in summer than in winter
This is because of Earth’s revolution around the sun
Earth’s dark side faces a slightly different direction in space each night

26. Earth’s Orbital Motion: Seasonal Changes
Because of Earth’s motion, the sun appears (to an observer on Earth) to move relative to the background stars over the course of a year
This path is called the ecliptic, and it is the plane of Earth’s orbit around the sun
Zodiac – the 12 constellations through which the sun passes as it moves along the ecliptic

27. Graphic: http://www.astrologyclub.org/articles/ecliptic/ecliptic2.gif

28. Earth’s Orbital Motion: Seasonal Changes
Why do we have seasons?
Student answers
We have seasons because Earth is tilted on its axis.
The northern hemisphere faces towards the sun in summer, so the suns rays are more direct, and heat the Earth more.
The northern hemisphere faces away from the sun in winter, making the suns rays more glancing and less efficient at warming the Earth

29. Image: http://www.ifa.hawaii.edu/~barnes/ast110_06/ea/0427a.jpg

30. Earth’s Orbital Motion: Seasonal Changes
Summer solstice – point on the ecliptic where the sun is at its northernmost point above the celestial equator
This is the point in Earth’s orbit where the north pole points closest to the sun
This occurs on or near June 21, and corresponds to the longest day of the year in the northern hemisphere (shortest day in the southern hemisphere)

31. Earth’s Orbital Motion: Seasonal Changes
Winter solstice – point on the ecliptic where the sun is at its southernmost point
Occurs on or near December 21, and corresponds to the shortest day of the year (in the northern hemisphere)

32. Earth’s Orbital Motion: Seasonal Changes
Equinoxes – the two points where the ecliptic intersects the celestial equator
On these dates, day and night are of equal length

33. Earth’s Orbital Motion: Seasonal Changes
Autumnal equinox – fall: on or near September 21
The sun crosses from the northern to the southern hemisphere
Vernal equinox – spring: on or near March 21
Sun crosses celestial equator moving north

34. Image: http://www. ecology

35. Earth’s Orbital Motion: Seasonal Changes
Tropical year – interval of time from one vernal equinox to the next = mean solar days
This is the year that our calendar measures

36. Earth’s Orbital Motion: Long Term Changes
Earth spins on its axis, travels around the sun and moves with the sun through our galaxy

37. Earth’s Orbital Motion: Long Term Changes
Precession – Earth’s axis changes direction over the course of time
Caused by torques (twisting forces) on Earth due to the gravitational pulls of the moon and sun
These forces affect our planet in a way that is similar to the way the torque due to Earth’s gravity affects a top
A complete cycle of precession takes about 26,000 years and traces out a cone

38. Image: http://www.lcsd.gov.hk/
CE/Museum/Space/Education
Resource/Universe/framed_e/
lecture/ch03/imgs/precession.jpg

39. Earth’s Orbital Motion: Long Term Changes
Sidereal year – time required for Earth to complete exactly one orbit around the sun relative to the stars = mean solar days
The sidereal year is ~20 minutes longer than the tropical year
The reason for this difference is precession

40. The Measurement of Distance
Triangulation – distance-measurement method based on principles of Euclidian geometry
Forms foundation of family of distance-measurement techniques that make up the cosmic distance scale

41. The Measurement of Distance
If we know the value of one side of a triangle (its baseline) and two of its angles, we can calculate the lengths of the remaining sides of the triangle
Close objects are measured this way using parallax – an object’s apparent shift relative to some more distant background as observer’s point of view changes
Earth’s orbit is used as the baseline to measure the angle through which line of sight to an object shifts

42. Image: http://www.astro.gla.ac.uk/users/kenton/C185/parallax.gif