Another Way to Measure the Size of the Universe 125
Another Way to Measure the Size of the Universe:
ATriangulation Trick
The distance to the Moon can be calculated by making a baseline between two spots on
Earth and using triangulation (see pages 68–69). However, this method cannot be used
to calculate the distance to the Sun, since that distance is too great. The mean distance to
the Sun is approximately 150 million km (93 million miles), which is approximately 12,000
times the diameter of Earth. A slight measurement error will have a significant effect, and
the length of the baseline that can be taken on Earth is limited. To calculate the distance
to the Sun, we would need a spot on Earth and a spot somewhere off of Earth to create the
baseline.
Luckily, we can use the distance to the Moon, which we discovered by triangulating
from a baseline on Earth, as the baseline for triangulating the distance to the Sun. If this
were ancient Greece, Aristarchus would help us out here because he already told us the
Moon-Earth-Sun angle is 87°. But this isn’t ancient Greece.
Since the angle created by the Moon and Sun was incorrectly measured to be 87°,
according to the surveying technology available at the time, Aristarchus concluded that the
distance to the Sun was approximately 20 times farther than the distance to the Moon. With
the help of current technology, we now know it is approximately 390 times further. Never-
theless, this method thought up by Aristarchus more than 2,300 years ago is really quite
remarkable.
Earth
87˚ (Actually 89.85˚)
Sun
Moon
Mean distance between Earth and the Sun: 150 million km (93 million miles)
Equatorial radius of Earth:
6,378 km (3,963 miles)
Method of finding the distance between Earth and the Sun

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