Post on 03-Jul-2018
Stars are named by a Greek letter (…) according to their relative brightness within a given constellation plus the
possessive form of the name of the constellation:
Betelgeuse = Orionis,
Rigel = Orionis
Betelgeuse
Rigel
Constellations
The Celestial Sphere• Zenith = point on
the celestial sphere directly overhead
• Nadir = point on the c. s. directly underneath (not visible!)
• Celestial equator = projection of the Earth’s equator onto the c. s.
• North celestial pole = projection of the Earth’s north pole onto the c.s.
The Celestial Sphere
On the sky, we measure distances between objects as angles:The full circle has 360o (degrees),1o has 60’ (arc minutes),1’ has 60” (arc seconds).
Degrees & Arcseconds
There are 360° in a full circle.
Arcminutes: 1° = 60’Arcseconds: 1’ = 60”
Extended fist = 10°
Ɵ
Ɵ = 31 arcminutes
Brainstorm!1) How many arcseconds?
a. 1° b. 1/2° c. 2° d. 2’
2) Through how many degrees does the Earth rotate in 1 hour?
The Celestial Sphere (II)• From geographic
latitude l (northern hemisphere), you see the celestial north pole ldegrees above the horizon;
• From geographic latitude –l(southern hemisphere), you see the celestial south pole ldegrees above the horizon.
• The celestial equator culminates 90o – l above the horizon.
l
90o - l
Example:New York City: l ≈ 40.70
HorizonNorth
Celestial North Pole
40.7°
South
49.3°
Celestial Equator
The Celestial South Pole is not visible from the northern hemisphere.
Horizon
Apparent Motion of the Celestial Sphere
Looking north, you see stars circling counterclockwise around the celestial north pole.
Precession (I)
• Gravity is pulling on a slanted top => wobbling around the vertical.
• The Sun’s gravity is doing the same to the Earth.
• The resulting “wobbling” of the Earth’s axis of rotation around the vertical with respect to the Ecliptic takes about 26,000 years and is called precession.
Precession (II)
• 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.
• ~12,000 years from now, it will be close to Vega in the constellation Lyra.
There is nothing peculiar about Polaris at all (it is neither particularly bright nor nearby, etc.).
Brainstorm!1)Why do stars appear
to move east-west?
2)Does precession have any effect on the celestial poles or celestial equator?
3) How many rotations does Earth complete in 1 revolution?
4) What are the Sun’s motions?
The Magnitude ScaleFirst introduced by Hipparchus
(160–127 B.C.):
• Brightest stars: ~1st magnitude• Faintest stars (unaided eye): 6th magnitude
More quantitative:
• 1st magnitude stars appear 100 times brighter than 6th magnitude stars
• 1 magnitude difference gives a factor of2.512 in apparent brightness (larger magnitude => fainter object!)
Example:
Betelgeuse
Rigel
Magnitude = 0.41 magMagnitude Difference
Flux Ratio
1 2.5122 2.512 × 2.512 =
(2.512)2 = 6.31… …5 (2.512)5 = 100
Magnitude = 0.14 mag
For a magnitude differenceof 0.41 – 0.14 = 0.27, we
find an flux ratio of(2.512)0.27 = 1.28
The magnitude scale system can be extended towards negative numbers (very bright) and numbers > 6 (faint
objects):
Sirius (brightest star in the sky): mv = –1.42
Full Moon: mv = –12.5
Sun: mv = –26.5
The Magnitude Scale
Brainstorm!1) Which would appear
fainter to us when viewed from Earth with our eyes?
a. −4 magnitude starb. +4 magnitude starc. 0 magnitude star
2) If two stars have the same energy output, what would make one star appear fainter than the other, when viewed from Earth?
The Annual Motion of the Sun
• Due to Earth’s revolution around the Sun, the Sun appears to move through the zodiacal constellations.
• 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.
The Seasons (I)
• The Earth’s equator is inclined against the ecliptic by 23.5°.
• The different incidence angle of the Sun’s rays causes the seasons on Earth.
The Seasons (IV)
SunEarth in July
Earth in January
The Earth’s distance from the Sun has only a very minor influence on seasonal temperature variations.
Earth’s orbit (eccentricity greatly exaggerated)
Astronomical Influences on Earth’s Climate (I)
Factors affecting Earth’s climate:
• Eccentricity of Earth’s orbit around the Sun (varies over period of ~100,000 years)
• Precession (period of ~26,000 years)
• Inclination of Earth’s axis versus orbital plane
Milankovitch Hypothesis: Changes in all three of these aspects are responsible for long-term global climate changes (ice ages).
Astronomical Influences on Earth’s Climate (II)
Last glaciation
End of last
glaciation
Polar regions receiving less than average energy
from the Sun
Polar regions receiving more than average energy from
the Sun
Brainstorm!1) Refer to your
celestial sphere map. Locate the position of the Sun for each season.
2) Will the seasons on Earth ever change? Explain.
3) At what latitude is the Sun directly overhead, at noon, on the first day of our “summer”? What is this latitude called?
4) At what latitude is the Sun directly overhead, at noon, on the first day of our “winter”? What is this latitude called?
Motions of the Planets
Earth
VenusMercury
All planets in almost circular (elliptical) orbits around the Sun, in approx. the same plane
(ecliptic)
Sense of revolution:
counter-clockwise
(distances and times reproduced to scale)
The Moon is orbiting Earth in almost the same plane (ecliptic)
Mercury appears, at most, ~28° from the Sun.
•It can occasionally be seen shortly after sunset in the west or before sunrise in the east.
Venus appears, at most, ~46° from the Sun.
•It can occasionally be seen for a few hours (at most) after sunset in the west or before sunrise in the east.
Apparent Motion of the Inner Planets
The Phases of the Moon (I)
As the Moon orbits around Earth, we see different portions of the Moon’s surface lit by the Sun, causing the phases of the Moon.
The Tidally-Locked Orbit of the Moon
The Moon is rotating with the same period around its axis as it is orbiting Earth (tidally locked).
We always see the same side of the Moon facing Earth.
Synodic vs SiderealSynodic motion
relates to an Earth rotation/revolution which results in the same view of the Sun or Moon
Sidereal motion is the process of returning to the same position with respect to the background stars
http://www.skywise711.com/Skeptic/Sidereal/sidereal.html
The Month is based on the motion of the Moon.
One synodic month = 29.53 days (complete cycle moon phases)
One sidereal month = 27.3 days (one moon orbit)
A DayThe day is based on the rotation of Earth.
A sidereal day = 23 h, 56m, 4.09 s A solar day = 24 hours
1 → 2Earth Observer re-
points to distant star (sidereal)
Earth rotates 360°
1 → 3Earth Observer re-
points to sun (synodic)
Brainstorm!
1) In general, what does sidereal time measure?
2) Why do we have leap year?
3) What is the change in position of the moon in the sky, in 24 hours? (Number of degrees)
Lunar Eclipses• Earth’s shadow
consists of a zone of full shadow, the
umbra, and a zone of partial shadow, the
penumbra.
• If the Moon passes through Earth’s full
shadow (umbra), we see a lunar eclipse.
• If the entire surface of the Moon enters
the umbra, the lunar eclipse is total.
A Total Lunar Eclipse (II)
• A total lunar eclipse can last up to 1 hour and 40 minutes.
• During a total eclipse, the Moon has a faint, red glow, reflecting sunlight scattered in the Earth’s atmosphere.
Brainstorm!1) What is the phase of the Moon during a total
lunar eclipse?
2) Why is the shadow dark and then red, during a total lunar eclipse?
3) How often can an eclipse occur?
4) Which are more commonly seen, solar or lunar eclipses? Why?
Solar Eclipses (I)
The angular diameter of the Moon (~0.5o) is almost exactly the same as that of the Sun.This is a pure coincidence. The Moon’s linear diameter is much smaller than that of the Sun.
Solar Eclipses (II)
Due to the equal angular diameters, the Moon can cover the Sun completely when it passes in front of
the Sun, causing a total solar eclipse.
Total Solar Eclipse
The Moon’s shadow sweeps across the Earth, over points
from where we can see a solar eclipse.
Total Solar Eclipse
During a total solar eclipse, the solar chromosphere, corona, and prominences can be seen.
Earth’s and Moon’s orbits are slightly elliptical:
Sun
Earth
Moon
(Eccentricities greatly exaggerated!)
Perihelion = position closest to the sun
Aphelion = position
furthest away from
the Sun
Perigee = position closest to Earth
Apogee = position furthest away from
Earth
Annular Solar EclipsesThe angular sizes of the Moon and the Sun vary, depending on their distance from Earth.
When the Earth is near perihelion, and the Moon isnear apogee, we see anannular solar eclipse.
Perigee Apogee Perihelion Aphelion
Very Important Warning:Never observe the Sun directly with your bare eyes, not even during a partial solar eclipse!
Use specially designed solar
viewing shades, solar filters, or a
projection technique.