1. PHYS Astronomy
Local Skies
Lines of constant declination cross the sky at different altitudes, depending on your location on Earth.
declination line = your latitude - goes through your zenith
the altitude of the N or S celestial pole = your latitude

2. PHYS Astronomy
Local Skies

3. PHYS Astronomy
Determining latitude
Find celestial pole - latitude equal to angular altitude - in northern hemisphere Polaris is within 1º of celestial pole
For more precision - use star with known declination - determine angular altitude as it crosses your meridian -imaginary half circle drawn from your horizon due south, through zenith (point directly overhead) to horizon due north - or when star is at its highest altitude in the sky. Ancients used cross-staff or Jacob’s ladder to determine angular altitude. Modern device called a sextant.
Sextant

4. PHYS Astronomy
Vega crosses your meridian in the southern sky at 78º 44’. You know it crosses your meridian at 38º 44’ north of the celestial equator. So the celestial equator must cross your meridian at an altitude of 40º so your latitude is 50º. The formula for latitude is
north/south of zenith. Sun can also be used if you know the date and the Sun’s declination on that date.

5. Annual Motion of the Sun
PHYS Astronomy
Annual Motion of the Sun
The R.A. of the Sun…
increases about 2 hours per month
The Declination of the Sun…
varies between –23º and +23º

6. Determining longitude
PHYS Astronomy
Celestial Navigation
Determining longitude
Need to compare current positions of objects in your sky with positions at known longitude - Greenwich (0º Longitude). For instance - use sundial to determine local solar time is 3:00 PM. If time at Greenwich is 1:00 PM, you are two hours east of Greenwich and your longitude is 15º X 2 = 30º East Longitude.
Accurate determination of longitude required invention of clock that could remain accurate on a rocking ship. By early 1700s, considered so important, British government offered large monetary prize for the solution - claimed by John Harrison in Clock lost only 5 seconds during a 9-week voyage.

7. PHYS Astronomy
Seasons occur because even though the Earth's axis remains pointed toward Polaris throughout the year, the orientation of the axis relative to the Sun changes as the Earth orbits the Sun. Around the time of the summer solstice, the Northern Hemisphere has summer because it is tipped toward the Sun, and the Southern Hemisphere has winter because it is tipped away from the Sun. The situation is reversed around the time of the winter solstice when the Northern Hemisphere has winter and the Southern Hemisphere has summer. At the equinoxes, both hemispheres receive equal amounts of light.

8. Why Does Flux Sunlight Vary Animation
PHYS Astronomy
Why Does Flux Sunlight Vary Animation

9. PHYS Astronomy
Antarctica
June 21
December 21

10. Why are the warmest days one to two months after summer solstice?
PHYS Astronomy
In the summer hemisphere, the sun follows a longer and higher path. The sunlight is more intense - more direct and more concentrated. In the winter hemisphere, the sun follows a shorter and lower path. The sunlight is less direct and less intense.
Why are the warmest days one to two months after summer solstice?

11. Why are the seasons more extreme in the Northern hemisphere?
PHYS Astronomy
Why are the seasons more extreme in the Northern hemisphere?

12. PHYS Astronomy
1. Most of Earth’s land mass in in the Northern Hemisphere. Water takes longer to heat or cool than soil or rock (water has a higher heat capacity). The water temperature remains relatively constant, thereby moderating the climate.
2. Earth is slightly farther from the sun during northern summer solsitce - moves slower in its orbit so summer/winter is days longer/shorter. This effect is more important then the slightly more intense sunlight due to Earth being closer/farther away.

13. Five Major Circles of Latitude
PHYS Astronomy
Five Major Circles of Latitude
1. The Arctic Circle (66.5 degrees N)
2. Tropic of Cancer (23.5 degrees N)
3. The Equator
4. The Tropic Capricorn (23.5 degrees S)
5. The Antarctic Circle (66.5 degrees S)
What is special about these latitude circles?

14. Five Major Circles of Latitude
PHYS Astronomy
Five Major Circles of Latitude
The Arctic and Antarctic Circles - One day a year the sun shines all day and one day a year it doesn’t shine at all.
Tropic of Cancer (Capricorn) - The sun is never directly overhead at higher latitudes.

15. PHYS Astronomy
Precession
(a) A spinning top slowly wobbles, or precesses, more slowly than it spins. (b) The Earth's axis also precesses. Each precession cycle takes about 26,000 years. Note that the axis tilt remains about the same throughout the cycle, but changing orientation of the axis means that Polaris is only a temporary North Star.

16. PHYS Astronomy
Precession Movie

17. Gravitational Attraction
PHYS Astronomy
Gravitational
Attraction
The Sun’s gravity (and the Moon’s to a lesser degree) tugs on the Earth trying to straighten out its rotational axis. However, like any rotating object, the Earth tends to keep spinning around the same axis. The result is that gravity succeeds only in making the axis precess.

18. Climate Changes 41,000 yrs 26,000 yrs 100,000 yrs
PHYS Astronomy
Climate Changes
26,000 yrs
41,000 yrs
100,000 yrs
Changes in Earth’s orbit and orientation cause cyclic changes in climate - ice ages. Mildest period about 5,000 years ago - headed for another ice age.

19. PHYS Astronomy
Milankovitch Theory
Variations in Earth's orbit, the resulting changes in solar energy flux at high latitude, and the observed glacial cycles.
Milankovitch Theory - precession of equinoxes, variations in tilt of Earth's axis (obliquity) and changes in eccentricity of the Earth's orbit responsible for observed 100 kyr cycle in ice ages by varying amount of sunlight received by the Earth particularly noticeable in high northern latitude summer.

20. Aspects of an Inferior Planet.
PHYS Astronomy
Aspects of an Inferior Planet.
Various configuration of an inferior planet are defined as shown here. Any planet whose orbit is smaller than Earth’s orbit is called a “inferior” planet. Mercury and Venus are the only inferior planets.

21. PHYS Astronomy
Mercury
As viewed from Earth, Mercury can be seen only near times of greatest eastern or western elongation.
At greatest western elongation (when the planet is farthest west of the sun in the sky), Mercury rises about 1 1/2 hours before sunrise.
At greatest eastern elongation (when the planet is farthest east of the sun in the sky), Mercury sets about 1 1/2 hours after sunset.

22. PHYS Astronomy
Venus
Venus is at maximum elongation at 47° - at maximum brilliancy at 39º - combination of amount of sunlit Venus visible and distance from Earth - brightness of reflected light

23. PHYS Astronomy
Venus and Mercury can sometimes be seen in the west at sunset - “evening stars” or in the east at sunrise - morning “stars”.

24. Planetary Transits Mercury’s orbit tilted 7° to ecliptic plane
PHYS Astronomy
Planetary Transits
Mercury’s orbit tilted 7° to ecliptic plane
- Crosses ecliptic plane only at the two nodes
- Transit of Mercury across Sun’s disk possible only when Mercury passes through inferior conjunction near nodes - around May 8 and Nov 10 - occurs every years. Next on May 9, 2016.

25. Mercury Transit of the Sun
PHYS Astronomy
Mercury Transit of the Sun

26. Images taken at UTD of Mercury transit of November, 2006
PHYS Astronomy
Images taken at UTD of Mercury transit of November, 2006

27. Venus Transits Venus orbit tilted 3.4º to the ecliptic
PHYS Astronomy
Venus Transits
Venus orbit tilted 3.4º to the ecliptic
Transits of Venus only occur in pairs about 8 years apart every ~120 years.
Transits occurred in and (June)
Edmund Halley one of few to observe first transit in 1677 after invention of telescope - called upon future astronomers to observe subsequent transits.
On his first voyage, Captain Cook traveled to Tahiti to observe transit of 1769 in an attempt to estimate the distance from the Earth to the Sun using triangulation and the parallax effect.
Measurements not very accurate:
- intense sunlight filtering through Venus' atmosphere fuzzed the edge of the disk and decreased the precision with which Cook could time the transit
- his measurements disagreed with those of ship's astronomer Charles Green, who observed the transit beside Cook, by as much as 42 seconds.

28. PHYS Astronomy
- Cook and Green also observed the "black drop effect." When Venus is near the limb of the sun - the critical moment for transit timing - the black of space beyond the Sun's limb seems to reach in and touch the planet - made it hard to say just when the transit began or ended.
- a problem for observers elsewhere - observations of Venus' 1769 transit from 76 points around the globe not precise enough to set the scale of the solar system. Astronomers didn't manage that until the 19th century when they used photography to record the next pair of transits.

29. Venus Transit of the Sun
PHYS Astronomy
Venus Transit of the Sun

30. Aspects of a Superior Planet.
PHYS Astronomy
Aspects of a Superior Planet.
Various configurations of superior planet are defined as shown here. Any planet whose orbit is larger than Earth’s orbit is called a “superior” planet.

31. PHYS Astronomy