Transit of Venus

Discussion Point – Transit of Venus (1769)

This discussion point links to Years 5 and 7 ‘Science’. These curriculum links are listed at the bottom of the page.

How big is the solar system? The galaxy? The universe? We have a greater idea now than we did in 1768, but we still don’t really know. The 1769 transit of Venus was important because it helped us start to measure the size of the universe.

How big is the universe?

The universe is about 93 billion light years across, but it is continually expanding.

What is a light year? It is the distance that light can travel in a year. A light year is a distance measurement, not a time measurement.

Measuring the universe

A transit of Venus is when Venus crosses in front of the Sun. The 1769 transit of Venus was important because it helped us measure the size of the universe. If the time that it took Venus to cross in front of the Sun could be measured, then the distance between the Sun and the Earth could be calculated, which could give key information to finding out the size of the Universe.

The distance of each planet from the Sun was determined in 1619. The figures were all based on the distance between Earth and the Sun, which was determined to be 1 astronomical unit (AU). The transit of Venus was used to determine the numerical value of 1 AU, and therefore the distance between Earth and other astronomical objects.

Watching the transit of Venus from multiple different locations helped us determine the distance between Earth and the Sun using trigonometry (the study of mathematics relating to triangles). The underlying theory for this method is parallax – the shift in position that occurs when viewing an object from two different points.

Parallax

The parallax effect relies on three main factors:

an object being focused on
a stationary background object
two separate viewpoints.

When you look at the same object from different viewpoints, the object’s location appears to change. This only happens if you are not directly in line with the object. This apparent change in location is especially noticeable when compared to distant, background objects. This apparent change in location is referred to as parallax.

Method and Results

The exact time at which Venus was seen at four specific different locations in front of the sun was recorded: when it touched the edge of the Sun, when it was completely within the disc of the Sun, when it touched the opposite edge and when it had completely left the disc of the Sun. From these figures the time it took for the transit of Venus to occur was calculated.

Observing Venus at different locations around the Earth created the different viewpoints need for the parallax effect to occur.
The three essential factors for parallax were:

an object being focused on: Venus
a stationary background object: The Sun
two separate viewpoints: different geographical locations

At two different geographical locations Venus would appear to cross the sun at different points; Venus’ transit would appear to take different paths. These different paths appear to take different lengths of time. Comparing the different lengths of time it took Venus to cross in front of the Sun allowed the scientists to calculate one astronomical unit.

On their own, HMB Endeavour’s results were not conclusive. After HMB Endeavour returned to England all the different results from the different sites around the world were merged; this created clearer results. This collaboration allowed us to approximate the value of 1 AU. When compared to current measurements, these results were surprisingly accurate (1 AU ≈150 million km).

Black drop effect

The largest cause of error when measuring the transit of Venus was the ‘black drop effect’. Venus appeared fuzzy to the scientists. When observing the 1761 transit of Venus, scientists thought that with good telescopes they would be able to see the black disc of Venus moving across the bright sun. Between 1761 and 1769 a lot of research and work went into improving telescopes. However, when Venus crossed the sun’s border in 1769 its edges still appeared fuzzy, much to the frustration of the scientists. This meant it was hard to determine the exact times of Venus’ transit.

This is still a problem today and is now thought to be due to three main reasons:

the Sun’s atmosphere, which creates a haziness when viewing these types of transits
diffraction – the way light bends when it goes through a tiny space.
quality of telescopes – our imaging of the transit of Venus is getting better with every transit.

Activity

Activity – Diffraction

Instructions

Find a source of light.
Hold your hand up towards the light and hold your index finger and thumb close until they are about 1 cm apart.
Look at the light through the space between your thumb and index finger
Slowly close the 1 cm gap. Do you notice that your fingers appear to be touching before they actually touch?

Explanation
Your fingers appear to be touching because of diffraction. Diffraction is when a wave bends around an object or travels through a small slit. This phenomenon is often talked about in light but is also seen in ocean waves. The amount of diffraction that occurs depends on the size of slit compared to the wave’s wavelength.
In this case the light is bouncing around your fingertips and causing distortion. This makes the edges of your fingers appear fuzzy.

Activity – Parallax

Instructions

Hold your finger out in front of your face and close your right eye. Look at where your finger appears to be compared to things in the background.
Then quickly open the right eye and close the left (always keep one eye open). Do this a few times. Look at where your finger appears to be compared to things in the background.

Explanation
Your finger appears to be in different positions because of parallax. Your eyes give you two different viewpoints, and you are comparing your finger to any objects in the background. Even though your eyes are in only slightly different positions, it is enough to create a parallax effect.

The parallax effect relies on three main factors:

an object being focused on
a stationary background object
two separate viewpoints.

When you look at the same object from different viewpoints, the object’s location appears to change. This only happens if you are not in line with the object. This apparent change in location is especially noticeable when compared to distant, background objects.

The three necessary factors in this example are:

an object being focused on: your finger
a stationary background object: the objects behind your finger
two separate viewpoints: your two eyes

Your brain uses this apparent difference in location to give you depth perception, and it is also the principle that 3D videos are based on.

Note that it would be quite challenging to do the maths required on this parallax effect; the bigger the geographical distance between the two viewing sites, the better. This is why scientists travelled all over the world in 1769.

Australian National Curriculum Links

Year 5

Science
Science Understanding

The Earth is part of a system of planets orbiting around a star (the sun) (ACSSU078)
Light from a source forms shadows and can be absorbed, reflected and refracted (ACSSU080)

Year 7