1. How Eclipses Occur, and the 2017 Great American Solar Eclipse
Dr. Bill Blair
JHU Quarknet Presentation
July 25, 2017
(with thanks to the STScI Office of Public Outreach for some of the graphics)

2. Eclipses through History...

3. Moon Phases and Orbit NOAO
Credit: T.A.Rector, I.P.Dell'Antonio, NOAO/AURA/NSF
NOAO

4. SUN
Synodic Month = days

6. Lunar Phases and Geometry

7. Lunar Eclipse Sun Earth Moon
Approx. Relative Size of Earth and Moon: Moon fits in earth’s shadow with room to spare!

9. Lunar eclipse – visible from anywhere on the night-side of earth

10. Lunar Eclipse Time Series

11. Penumbra
Umbra

12. Why is the moon red in Lunar Eclipse?

13. Solar Eclipse Sun Earth Moon
Angular Size of Sun and Moon are nearly the same! (Only a small portion of the sunlit side of earth can see, however.)

15. ...but of course not to scale!
This is the geometry...
What else is wrong with this picture?
...but of course not to scale!

16. Earth’s diameter (8000 miles) is about
Earth’s diameter (8000 miles) is about
4 times the Moon’s diameter (2200 miles)

17. The Moon is located about 240,000 miles (30 Earth diameters) away.
(Now think about the shadows needed to cause eclipses!)
(Yardstick demo)

18. (sun)

19. Why Not Eclipses Every Month?
Eclipse Ingredients
Moon Phase
Full moon (lunar) or new moon (solar)
Orbital plane alignment
Moon crossing Earth-Sun plane
Earth-Moon Distance (apogee, perigee, or inbetween)
Affects apparent (angular) size of moon

20. Moon’s orbital plane is tipped ~5o to Earth’s
Need full or new moon to occur along the intersection of the orbit planes for eclipses!

21. Otherwise the shadows miss their targets!

22. Moon’s orbit is slightly elliptical, so its angular size changes during its orbit.
a > b a b

23. Anthony Ayiomamitis

24. Annular Eclipse (February 2017)

25. Annular Solar Eclipse Views

26. Solar Eclipse Phenomenology
NASA

28. Partial eclipse showing large sunspot group
Partial phases last about 3 hours total. (Totality is in the middle of this time if you are on the center line.)

29. Partial eclipse showing large sunspot group
Earth to scale

30. Diamond Ring Effect

31. “Baily’s Beads”
(Sunlight passing through lunar valleys on the moon’s limb)
Pierre Arpin

32. Baily’s Beads Time Series
Richard Nugent

33. Total Eclipse showing solar corona
Klaus Kemmerich

34. Coronal structure depends on solar activity level
HAO / NCAR

35. Solar Chromosphere and Prominences
Luc Viatour

36. A Realistic Naked Eye View...

37. Solar Eclipse Viewed from Space

38. March 9 2016 Pacific Solar eclipse captured by DISCOVR [Deep Space Climate Observatory] (NASA)
DSCOVR / NASA

39. March 9 2016 Pacific Solar eclipse captured by DISCOVR [Deep Space Climate Observatory] (NASA)
DSCOVR / NASA

40. March 9 2016 Pacific Solar eclipse captured by DISCOVR [Deep Space Climate Observatory] (NASA)
DSCOVR / NASA

41. March 9 2016 Pacific Solar eclipse captured by DISCOVR [Deep Space Climate Observatory] (NASA)
DSCOVR / NASA

42. March 9 2016 Pacific Solar eclipse captured by DISCOVR [Deep Space Climate Observatory] (NASA)
DSCOVR / NASA

43. March 9 2016 Pacific Solar eclipse captured by DISCOVR [Deep Space Climate Observatory] (NASA)
DSCOVR / NASA

44. March 9 2016 Pacific Solar eclipse captured by DISCOVR [Deep Space Climate Observatory] (NASA)
DSCOVR / NASA

45. March 9 2016 Pacific Solar eclipse captured by DISCOVR [Deep Space Climate Observatory] (NASA)
DSCOVR / NASA

46. March 9 2016 Pacific Solar eclipse captured by DISCOVR [Deep Space Climate Observatory] (NASA)
DSCOVR / NASA

47. March 9 2016 Pacific Solar eclipse captured by DISCOVR [Deep Space Climate Observatory] (NASA)
DSCOVR / NASA

48. March 9 2016 Pacific Solar eclipse captured by DISCOVR [Deep Space Climate Observatory] (NASA)
DSCOVR / NASA

49. August 21, 2017 – Where Will You Be?

51. Total Solar Eclipse – August 21, 2017
Greatest eclipse at 18:25 UT
Duration 2 min 40 sec
Totality path is 71 miles wide
Partial eclipse across North America

53. Eclipse calculator by city: fractional coverage and timing:
Observing the Eclipse
Eclipse calculator by city: fractional coverage and timing:

54. Eclipse calculator by city: fractional coverage and timing:
Observing the Eclipse
Eclipse calculator by city: fractional coverage and timing:

55. Safety First Looking directly at the Sun is unsafe! Only one exception
Use certified solar filters, or eclipse glasses
SUNGLASSES ARE NOT ENOUGH!
Only one exception
Can look directly at Sun during total eclipse
Don’t look directly at Sun during partial eclipse!

56. Indirect Viewing (Partial phases)

57. For pinhole cameras, the image is inverted.

58. Use what you have available to create your own pinhole cameras….
Lucia Garcia

59. The Eclipse of 1919 and General Relativity

60. General Relativity 1915 – Einstein: “mass warps space”
Light that passes near the Sun will bend
Background star positions should shift slightly due to the mass of the sun

61. (Effect is greatly exaggerated in this example!)

62. Background stars could not be seen or measured until the sun was blocked!

63. General Relativity Tested
1915 – Einstein “mass warps space”
Light that passes near the Sun will bend
1919 – Expeditions to measure star positions during total solar eclipse and compare to earlier pictures of the field.
Result: Deviations matched predictions
General Relativity passed eclipse test!

64. Gravitational lens example as observed with Hubble

65. August eclipse planets and stars

66. Questions?