Astronomical distances The SI unit for length, the meter, is a very small unit to measure astronomical distances. There units usually used is astronomy: The Astronomical Unit (AU) – this is the average distance between the Earth and the Sun. This unit is more used within the Solar System. 1 AU = 1.5x10 11 m
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Astronomical distances The SI unit for length, the meter, is a very small unit to measure astronomical distances. There units usually used is astronomy:
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Astronomical distancesAstronomical distancesThe SI unit for length, the meter, is a very small unit to measure astronomical distances. There units usually used is astronomy:
The Astronomical Unit (AU) – this is the average distance between the Earth and the Sun. This unit is more used within the Solar System. 1 AU = 1.5x1011 m
The SI unit for length, the meter, is a very small unit to measure astronomical distances. There units usually used is astronomy:
The Astronomical Unit (AU) – this is the average distance between the Earth and the Sun. This unit is more used within the Solar System. 1 AU = 1.5x1011 m
Astronomical distancesAstronomical distances
The light year (ly) – this is the distance travelled by the light in one year. The light year (ly) – this is the distance travelled by the light in one year.
1 ly = 9.46x1015
m
c = 3x108 m/st = 1 year = 365.25 x 24 x 60 x 60= 3.16 x 107 s
Speed =Distance / Time
Distance = Speed x Time = 3x108 x 3.16 x 107 = 9.46 x 1015
m
Astronomical distancesAstronomical distances
The parsec (pc) – this is the distance at which 1 AU subtends an angle of 1 arcsencond.
The parsec (pc) – this is the distance at which 1 AU subtends an angle of 1 arcsencond.
1 pc = 3.086x1016
m
or
1 pc = 3.26 ly
““ParsecParsec” is short for” is short forparparallax arcallax arcsecsecondond
(206,000 times further than the Earth is from the Sun)
ParallaxParallax
Angle star/ball appears to
shift
“Baseline”
Distance to star/ball
Where star/ball appears relative to background
Space
Parallax is the change of angular position of two observations of a single object relative to each other as seen by an observer, caused by the motion of the observer.
Parallax is the change of angular position of two observations of a single object relative to each other as seen by an observer, caused by the motion of the observer.
ParallaxParallax
Baseline – R(Earth’s orbit)
Dis
tanc
e to
S
tar
- d
Parallax - p(Angle)
We know how big the Earth’s orbit is, we measure the shift (parallax), and then we get the distance…
ParallaxParallax
ParallaxParallax
For very small angles tan p ≈ p
(Distance) d
(Baseline) R (Parallax) tan p
d
R p
In conventional units it means that
m 10 x 3.986 m
36001
3602
10 x 1.5 pc 1 16
11
ParallaxParallax
arcsecond) ( p
1 (parsec) d
m 10 x 3.986 m
36001
3602
10 x 1.5 pc 1 16
11
d
R p
p
R d
The farther away an object gets, the smaller its
shift.
Eventually, the shift is too small to see.
Parallax has its limitsParallax has its limits
Quick ReferenceQuick Reference 0.5 degree The width of a full Moon, as viewed from the Earth's surface, is about
0.5 degree. The width of the Sun, as viewed from the Earth's surface, is also about 0.5 degree.
1.5 degrees Hold your hand at arm's length, and extend your pinky finger. The
width of your pinky finger is about 1.5 degrees. 5 degrees Hold your hand at arm's length, and extend your middle, ring, and
pinky fingers, with the three fingers touching. The width of your three fingers is about 5 degrees.
10 degrees Hold your hand at arm's length, and make a fist with your thumb
tucked over (or under) your other fingers. The width of your fist is about 10 degrees.
20 degrees Hold your hand at arm's length, and extend your thumb and pinky
finger. The distance between the tip of your thumb and the tip of your pinky finger is about 20 degrees.
0.5 degree The width of a full Moon, as viewed from the Earth's surface, is about
0.5 degree. The width of the Sun, as viewed from the Earth's surface, is also about 0.5 degree.
1.5 degrees Hold your hand at arm's length, and extend your pinky finger. The
width of your pinky finger is about 1.5 degrees. 5 degrees Hold your hand at arm's length, and extend your middle, ring, and
pinky fingers, with the three fingers touching. The width of your three fingers is about 5 degrees.
10 degrees Hold your hand at arm's length, and make a fist with your thumb
tucked over (or under) your other fingers. The width of your fist is about 10 degrees.
20 degrees Hold your hand at arm's length, and extend your thumb and pinky
finger. The distance between the tip of your thumb and the tip of your pinky finger is about 20 degrees.
Parallax ExperimentParallax Experiment
Using the quick reference angles that I gave you determine how far something is away near your house based on the parallax method. Include a schematic to show the placement of all objects. Your schematic should include relevant distances and calculations.
Using the quick reference angles that I gave you determine how far something is away near your house based on the parallax method. Include a schematic to show the placement of all objects. Your schematic should include relevant distances and calculations.
Usually, what we know is how bright the star looks to us here on Earth…
Usually, what we know is how bright the star looks to us here on Earth…
We call this its Apparent Magnitude
“What you see is what you get…”
The Magnitude ScaleThe Magnitude Scale Magnitudes are a way of
assigning a number to a star so we know how bright it is
Similar to how the Richter scale assigns a number to the strength of an earthquake
Magnitudes are a way of assigning a number to a star so we know how bright it is
Similar to how the Richter scale assigns a number to the strength of an earthquake
This is the “8.9” earthquake off
of Sumatra
Betelgeuse and Rigel, stars in Orion with
apparent magnitudes 0.3 and 0.9
The historical magnitude scale…The historical magnitude scale… Greeks ordered
the stars in the sky from brightest to faintest…
…so brighter stars have smaller magnitudes.
Greeks ordered the stars in the sky from brightest to faintest…
…so brighter stars have smaller magnitudes.
Magnitude Description
1st The 20 brightest stars
2nd stars less bright than the 20 brightest
3rd and so on...
4th getting dimmer each time
5th and more in each group, until
6th the dimmest stars (depending on your eyesight)
Later, astronomers quantified this system.
Later, astronomers quantified this system.
Because stars have such a wide range in brightness, magnitudes are on a “log scale”
Every one magnitude corresponds to a factor of 2.5 change in brightness
Every 5 magnitudes is a factor of 100 change in brightness
(because (2.5)5 = 2.5 x 2.5 x 2.5 x 2.5 x 2.5 = 100)
Because stars have such a wide range in brightness, magnitudes are on a “log scale”
Every one magnitude corresponds to a factor of 2.5 change in brightness
Every 5 magnitudes is a factor of 100 change in brightness
(because (2.5)5 = 2.5 x 2.5 x 2.5 x 2.5 x 2.5 = 100)