1. Distances

2. Transit of Venus
A transit occurs when one object in space passes in front of another.
Venus transits the Sun twice each century
2004 and 2012
2117 and 2123
NASA – Solar Dynamics Observatory

3. Halley’s Method
The transit of Venus was used to measure the parallax distance to the Sun.
The time difference is equivalent to an angular change.
Defines the AU
Vermeer, Duckysmokton, Ilia

4. Sources of Parallax Geocentric parallax uses the earth as a base.
Take a photo two times in one night.
Heliocentric parallax uses the sun as a base.
Take a photo at two different seasons.

5. Shifting Stars The orbit of the earth is used as the base.
Near stars appear to move more than far stars.
site A
site B

6. Parsec An arc-minute is 1/60 of a degree.
A playing card on edge at arms length
An arc-second is 1/60 of an arc-minute.
A human hair thickness at 10 meters
Distance is related to the parallax angle.
one arc-second gives about 3.3 ly.
This parallax-second is defined as one parsec (pc).

7. Stellar Parallax A telescope photo of the stars will show the shift.
Arcturus 11.3 pc
ESA - Hipparcos

8. Hipparcos The ESA operated Hipparcos from 1989-1993.
Parallax measurement
0.001 arc-second accuracy
118,200 stars

9. Gaia The ESA launched Gaia in December 2013.
24 micro-arc-second accuracy
Human hair at 1000 km
The goal is to catalog 109 stars.