Lunar Parallax Demonstration Project
Pete Lawrence, Selsey, UK



What is parallax?

Parallax is best explained using a diagram (or two).

Diagram 1 shows two observers (Observer 1 and Observer 2) looking at a foreground object (a red ball) against a distant background (a number strip).

Observer 1's view of the object, with respect to the background, will be different from Observer 2's view. The further apart they are, the more pronounced the apparent shift of the foreground object with respect to the apparently fixed background.
 
 
 
Diagram 1
 
Diagram 2


In addition, the effect will also be shown for an observer viewing from above or below another as shown in Diagram 2. Observer 3 is above Observer 1 and 2.

Lunar Parallax

   
The background stars in the night sky are a long distance from Earth. The Moon, on the other hand, is a relatively close neighbour of the Earth. This gives observers on the Earth the potential to see parallax in 'action'. If, in Diagrams 1 and 2, the red ball is replaced with the Moon and the number strip with the stars in the night sky, it should be clear that if two (or more) observers can observe the Moon at exactly the same time from different positions on the Earth's surface, then it should be possible to see parallax.

The distance from the Earth to the Moon is approximately 400,000km (250,000 miles). The Earth's diameter is approximately 12,756km (7,926 miles). This would be the maximum distance possible between Observer 1 and Observer 2. In practice, geographical (and meteorological!) effects will mean that observers are separated by less than the possible maximum distance.

In addition, using the real figures for the Moon's distance and Earth's diameter, it is clear that the diagrams above are very much exaggerated.

Diagram 3 shows the Earth-Moon system more-or-less to scale. Note that the lines drawn from opposite sides of the Earth to the Moon have the maximum possible separation (12,756km/7,926 miles) and are therefore showing the maximum possible parallax angle.
 
Diagram 3
 


A Lunar Eclipse, Some Astrophotographers and The Internet...

In order to 'see' lunar parallax, at least two images of the Moon need to be taken (Observer 1 and Observer 2). These observers need to be able to capture an image of the Moon at exactly the same time. The image needs to show the Moon together with some of the background stars as a reference.

Although it's relatively easy to photograph the Moon, it's brightness normally makes capturing stars around it quite difficult. At times when the Moon is not that bright (e.g. when it's a crescent) it tends to be situated in bright twilight skies that will make faint stars difficult to capture.

At the time of a total lunar eclipse, the Moon's brightness is dimmed considerably (the degree of dimming varies between eclipses) giving an astrophotographer the opportunity that he/she needs to capture the Moon and the elusive star background that sits behind it.

On the night of November 8th/9th 2003, a total eclipse of the Moon was due. This was a good parallax eclipse because during mid-eclipse the Moon would be sitting high above the UK meaning that it would be visible in the USA and across most of Europe.

Using the communication potential of the internet, I decided to try and gather a set of images from as far apart as possible to show lunar parallax. This is the Lunar Parallax Demonstration Probject or LPDP for short.

Fate was not kind on this night and, following some beautifully clear night skies on previous nights, the night in question was covered in a blanket of cloud for much of Europe. However, astrophotographers are a tenacious group and results were obtained from a number of widely spaced locations.
 
 
Diagram 4


Diagram 4 shows the Earth as seen from the Moon at the approximate time of mid-eclipse. The marks on the Earth's surface represent the astrophotographers who were taking photographs to contribute to this project.



LPDP Bonus Results

Most of this project was driven from questions raised in various forums throughout the internet. Primarily "Is it possible to see lunar parallax using amateur equipment?". During the results processing stage an additional question was raised...

"Is it possible to see different faces of the Moon from different locations at exactly the same time?"

The second question is summarised by Diagram 5...
 
 
Diagram 5


In addition - would it be possible, assuming such a variation in views existed, to create a stereoscopic image of the Moon? In essence, this would be a Moon as seen, assuming the Earth to be a giant head with two eyes separated by, hopefully, thousands of kilometers.

Click here to see the results...
Click here for the 2004 update image...



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