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Solar Parallax
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Introduction
What Is Solar Parallax?
In general, the solar parallax can be understood as the difference in position of the Sun as
seen from the Earth’s center and from a point of observer’s location. If the sun is at the zenith
(directly overhead), the parallax is 0. The solar parallax reaches the maximum value when the
sun is seen on the horizon and is named the Horizontal Parallax or simply Parallax.
Solar parallax plays a very important role in some astronomic fields because it’s relating to
the distance of the Sun from the Earth. Knowing the solar parallax and the mean Earth radius
allows astronomers to calculate the AU (astronomical unit), the first, small step on the long road
of establishing the size – and thus the age – of the visible Universe.
Why Is It Difficult To Measure The Solar Parallax?
As the parallax is usually determined by the shift of the objects against the distant
background, it becomes difficult to measure the parallax of the sun because the sun is too bright
to see the background stars.
It is not possible even during the solar eclipse because the moon covers the entire sun,
including the edge. When the edge reappears, the eclipse is over and it turns be too bright again.
Moreover, unlike the stellar parallax, we can’t use the radar to measure directly the solar
parallax because the Sun has no solid surface to reflect the radar efficiently.
Review
Despite of all these problems, the requirement of measuring the size of the solar system and
the distance between celestial objects always forces the astronomers to find out the solutions.
• The first attempt was made by Aristarchus. He tried to determine the distance
from the Earth to the Sun by estimating the angular separation between the Sun and the
quarter-phase Moon. But his result is not accurate.
• Since the 17th century, Venus and Mars became the candidates for parallax
measurement. After many attempts, astronomers finally could obtain, at least, the
acceptable value of the solar parallax.
• Then, Eros was chosen to being used in the measurement of British Astronomer
Royal Sir Harold Spencer Jones.
• Other methods such as measurement speeds and using radar in the modern
astronomy made it more flexible to gain the accurate solar parallax.
Group 16 - Parallax
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