METHOD 1: TIMING VENUS' ENTRANCE AND DEPARTURE ON THE SOLAR DISC

by Trond Erik Hillestad

Venus will enter and leave the Sun at slightly different times, if observed from different parts of the world. Measurements of these times make it possible to calculate the distance from the Earth to the Sun.

1. EQUIPMENT The measurements must be performed using an astronomical telescope. You will need: A telescope on a sturdy mount and tripod. The mounting should have fine adjustments in both axis. Apart from this, the exact type of mount is of little concern A solar filter that can be placed safely in front of the telescope's objective. We strongly advice against using filters that are placed at the eyepiece. A watch that has been synchronized according to some official time source. Digital clocks are better suited than analogue ones, and the accuracy should be within a few seconds.

Avoid using solar filters that threads into the eyepiece. There is a great risk that they will crack or even explode, exposing the eye for intense, focused sunlight. In a worst case, this can cause instant blindness! (Hi-res versions: JPG / Adobe Illustrator EPS in CMYK.) Illustration by: Trond Erik Hillestad

We recommend using a high quality solar filter that can be mounted in front of the telescope's objective. A second method for observing the Sun with a telescope - the projection method - is explained in method 2. While suitable for many forms of observations, it is doubtful that the projection method will allow for the time accuracy needed for the distance calculations. Illustration by: astronomy.no

2. THE MEASUREMENT Use high magnification, but not so high that the image of the Sun becomes very blurred due to turbulence in the air. Monitor the Sun extra closely from a few minutes before Venus enters the solar disc, until the entire planet is inside the solar limb. You should also monitor the Sun from a few minutes before Venus starts to leave the Solar disc, until the entire planet is free of the Sun. Measure the time for these four contact points as accurate as you can. The most important ones are the time for second and third contact (explained below). During the time-span from the first to second contact, and from the third to the fourth contact, several very exciting phenomenas can be seen. However, these will also make it more difficult to get an accurate read-out of the time. Read more about this challenge below. This page gives some more practical advice on how to observe the Venus transit.

1. contact = Venus touches the solar disc. 2. contact = The entire planet is on the solar disc. 3. contact = Venus start to depart the solar disc. 4. contact = The entire planet has left the solar disc. (Hi-res versions: JPG /Adobe Illustrator EPS in CMYK.) Illustration by: Trond Erik Hillestad

3. CALCULATING THE DISTANCE OF THE SUN

4. CHALLENGES

a. The black-drop effect. The black-drop effect may become visible when Venus enters or leaves the solar disc. The enormous brightness difference between the dark side of Venus and the intrinsically bright solar disc causes Venus to apparently merge with the solar disc. When Venus, according to the calculated time tables, should be outside the solar disc, it may instead appear to "be stuck" in the Sun's limb. You may create your own black-drop effect by bringing two fingers gradually closer to each other, while at the same time having a bright object in the background. It should then look as the two fingers meet before the actually do. Read more.

Ill.: Astronomy.no

b. The atmosphere of Venus. Venus' atmosphere may be visible as a small glowing ring around the planet, as long as Venus is situated close to the Sun, but not in front of its disc. Once, this was the first evidence that Venus has an atmosphere. Sunlight is scattered in the planet's atmosphere, and makes it glow.
The visibility of the glowing ring will depend somewhat on your telescope and observing conditions. It will, if at all visible close to second and third contact, make your timing efforts more difficult, as it will be harder to see exactly when the planet is just inside the solar limb.

Venus' atmosphere may be visible as a glowing ring, when the planet is positioned just outside of the solar disc. (Hi-res versions: JPG / Adobe Illustrator EPS in CMYK.) Illustration by: Trond Erik Hillestad.

c. The atmosphere of the Earth. Earth's atmosphere is more or less a turbulent place. If we look at an astronomical object through a telescope, we will not only magnify the object, but also the turbulence. This may cause the object to apparently smear and wobble.
The effect is generally more prominent in areas of the world that are situated between two climatic zones, like Norway.
Thus, the solar limb will appear to "boil" when we look at the Sun with high magnification through a telescope. This will make it more difficult to obtain accurate timings during the Venus transit.
The timing for the second and third contact is exactly when the entire planetary disc has just entered the solar disc, and just as the entire planetary disc begins to leave to solar disc. Immediately prior to second contact, and immediately after third contact, a slim dark area will appear to break the solar limb. After second contact, and before third contact, the solar limb will shine brightly just outside the planet's disc.
Close to second and third contact, the slim solar limb will most likely appear and disappear several times. Try to find the mid-point of this in-and-out-of-view behaviour. This will be the time for the second and third contact.
The combination of the black-drop effect and the turbulence of the Earth's atmosphere may result in something like this:

Before 2nd contact.

Before 2nd contact.

2nd contact.

After 2nd contact.

Illustrations by: Trond Erik Hillestad.

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