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How far is the Sun? - Transit of Venus 8 June 2004 observations

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How I answered the question "How far is the Sun?"!

2004 Venus Transit measured Sun to Earth as 149596765km +/- 3543 km

This page is dedicated to my parents: who attempted to answer many of my questions about the Universe as best as they could, and pointed me in the right direction for all the others.

Thanks Mom & Dad!

A brief story of why I waited 34 years to see the transit

Landon at age 2
Landon Noll, Age 2

I am told that one of my first full sentences that I spoke as a small child (~age 2 in 1962) was when I asked the following question, while sitting at the dinner table watching a sunset:

How far is the Sun?
My parents used the book:
Questions Children Ask, by Edith and Ernest Bonhivert; Standard Education Society, Inc., Chicago 1960, p126
to give me the answer of ''about 93,000,000 miles''.

This answer prompted me to ask:

What I really was asking was ''How did they measure such a long distance to the Sun?''. I knew that 93,000,000 miles must be very far because my grandparents house in Oregon was 515 miles away and it took many hours to drive there. And the longest tape measure in our wood shop was only 100 feet! I reasoned that measuring all the way to the Sun would simply take too long. ;-)

No, I was not waiting for the Transit of Venus at age 2. The waiting would start 7 years later ...

Morrison Planetarium Projector California Academy of Sciences building Morrison Planetarium Projector in more detail
Morrison projector at the California Academy of Sciences

In the summer of 1970 when I watched a show at the Morrison Planetarium of the California Academy of Sciences (then located within Golden Gate Park in San Francisco). The show was about the Quest for the Solar Parallax.

During the talk, I learned how the Transit of Venus was used to measure the distance to the Sun. I was very pleased because for the first time, I had some idea of how Edmond Halley first proposed to measure it.

path of Venus across the Sun in 2004

The speaker said that we would have to wait until 2004 to see another Venus transit. As a child 9 years I was determined to see it!

8 June 2004 - Arcetri Astrophysical Observatory

Now shift forward some 34 years. I am on the TravelQuest - Sky and Telescope Northern Transit of Venus tour:
TravelQuest - Sky and Telescope Northern Italy Transit of Venus tour
Image: TravelQuest - Sky and Telescope Northern Italy Transit of Venus tour
To time the transit I made use of the following:

I observed the Transit on a terrace at the Arcetri Astrophysical Observatory overlooking the house in which Galileo was imprisoned during the final years of his life:

Overhead view of the Arcetri Astrophysical Observatory
Arcetri Astrophysical Observatory - click for a larger view

On the morning of 8 June 2004, the sky was clear and the early morning (quality of) seeing as good.

My Transit observing location was at:

Arcetri Observatory
43º 45.013'N ± 0.0005'
11º 15.275'E ± 0.0005'
175m ± 5m

A playback of the recording shows that there was a 5 second period when I believed that Venus had completely entered the Sun's disk (2nd contact). I used the mid-point of that period as my 2nd contact time:

second contact diagram
2nd contact time: 05h 39m 49s UTC ± 0.5s

The following results were reported to the ESO VT-2004 Observation Campaign. The VT-2004 method is rigorous method The rather rigorous VT-2004 method uses as input the defined (known) value of the AU (a constant), points of contact timing data along with observation location data of one or more observers, and a comparision of said data to that of the theoretical observer located at the center of the earth.

The term Astronomical Unit (AU) means:

mean distance between the Earth and the Sun
Today, the Astronomical Unit (AU) is a defined rather than measured value.

The Astronomical Unit is defined to be:

1 AU = 149,597,870.691 km

The results from the VT-2004 method using the GPS location of my Arcetri observation site and my 2nd contact time are as follows:

I use the term measured Sun to Earth because of the Eccentricity of Earth's orbit. The Earth travels around the Sun in a Ellipse, not a circle. So the actual distance on the day of the Transit was slightly more than 1 AU. My data has been normalized to a mean Solar distance.

The term solar parallax (Π) means:

the angle subtended by the mean equatorial radius of the Earth at a distance of 1 AU
The value of the solar parallax (Π) is defined as:
8.79414" (angle in arc seconds)

My measurements compare favorably to modern definitions:

  • AU error (ΔAU): 2437 km (0.0016%)
  • parallax error: (ΔΠ) 0.00010" (0.0011%)

Using the recording again: There was an 8 second period during with I believed that Venus had left the Sun's disk (3rd contact). I used the mid-point of that period as my 3rd contact time:

third contact diagram
3rd contact time: 11h 04m 30s UTC ± 0.5s

The 3rd contact time, using the same ESO VT-2004 method of calculation, yields:

The 3rd contact measurements also modern definitions:

The distance to the Sun

Combining my 2nd contact and 3rd contact calculations, I measured the following:
Distance to the Sun (AU): 149,596,765 km ± 3543 km (0.0237% uncertainty: 1 part in 42224)
and only 1106 km short of the defined AU

Solar parallax (Π): 8.79432" ± 0.00018" (0.0020% uncertainty: 1 part in 48856)

and only 0.00018" more than the defined (Π)

My observations fell right in the middle of the many observers who participated in the VT-2004 observation program:

Zoom on observer vs distance

Observation remarks

My intention was to view the Transit with an instrument that matched the recommendations of US Naval Observatory's 1882 transit program. For example, they recommended 3 to 6 inches in aperture and 150x to 200x in magnification. I wanted to duplicate their observing conditions using a modern optical system that fit within their 1882 program specifications.

The main question I wanted to answer was: How much of the transit effects I would be able to observe? Nobody alive today saw the 1882 transit. I wondered if my transit experience using modern optics would reproduce the effects reported in 1882.

Like the "canals of Mars", 19th century observers sometimes saw things that were not real. On the other hand most of their work was visual in those days. They were very skilled visual observers, perhaps much more than the average observer is today. Would modern optics and a more extensive knowledge of Venus change what I observed?

Here are some of the effects that I was looking for during the Transit:

I recommend that you read TRACE WHITE LIGHT INGRESS/EGRESS IMAGING for some interesting work with the TRACE spacecraft's observations of the Venus transit.

In conclusion:

Weather Forecasting

cloudless satellite image of europe

On the mornings prior to the transit, we out group started out moving on the bus, I would stand up and gave a transit weather forecast in 3 parts (Geographical, Statistical, and Literary):

Thank you

Thanks goes out to:

© 1994-2022 Landon Curt Noll
chongo (was here) /\oo/\
$Revision: 7.9 $ $Date: 2022/07/07 22:33:35 $