Who weighed
the Earth & where were they standing?
The Sunday Nation
Nairobi,
04 April 2010
Justin Osey Peter,
fires off three direct questions: “(1) What is weight and what is
gravity and are the two related? (2) Is it true that the bigger the
planet or body in space, the bigger the gravity it will attract? (3)
What is the weight of our galaxy the Milky Way since we know that of our
planet earth and how was it weighed?”
Weight is the
gravitational force felt by a smaller body when it is in the vicinity of
a much heavier one. For example, it is the pulling effect that we [our
small bodies] feel when we are near or on the surface of the Earth [the
larger one].
Weight is directly
related to the amount of gravity in the location where it is measured.
Therefore, it varies as one moves from place to place. For example: the
gravity is greater at the poles than at the equator; thus you would
weigh less at the Arctic than when you are in
The reason for this variation is that the amount of gravity reduces as one gets farther from the centre of the bigger body. Since the poles are close to the centre of the Earth than the equatorial regions, it turns out that objects are slightly heavier at the poles than at the equator.
The answer to the second question is both no and yes! No; because it is incorrectly phrased. Objects do not “attract gravity”; rather gravity exists spontaneously wherever there is a mass. Yes; because the greater the mass of an object, the larger the gravitational intensity. I have carefully avoided using the word “bigger” since that may also cause confusion. For example, a football is “bigger” than a shot-put ball, but the latter is much more massive.
Having clarified the definition of weight, it should now be apparent that the third question cannot be answered. The reason being that when stating the weight of a body, we must specify the larger object is providing the gravity. While on Earth, it is usually assumed that this planet is the source.
Clearly then, the question about the weight of the Earth cannot be answered because we don’t have a much larger body in the vicinity. As Archimedes once quipped: “show me a place to stand and I will move the Earth”. The same reasoning also applies to the weight of the Milky Way galaxy – it cannot be determined.
Their masses, however, is a different story altogether. That of the
Earth is known pretty accurately – 5,974 billion-billion tonnes. The
method of measuring is very simple; all you need is a simple pendulum, a
stop clock and some knowledge of
The mass of the galaxy, on the other hand is not so easy to determine; you need to estimate the number of stars and then determine their average mass. Estimates vary widely from 200 to 600 billion times the mass of our Sun. But still: who measured the mass of the Sun and where were they standing? Well; that is a story for another day.
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