“How high is the sky?” This is a question that has pondered many people over the centuries. Within that question lies many others such as, “How far are the planets?” or, “Why are the stars so far away?” When the sky is examined and these questions are asked, distances can be explained in terms of light years. A light year is the distance traveled by a light ray, at a speed of 300,000 kilometres per second over a span of one year. For example, the Sun is eight light minutes away.
Before light years were understood, observers, such as the Greeks, used parallax to estimate the distance to a planet or star. During the 17th century, Johannes Kepler constructed a scale model of the solar system by mapping positions of the planets relative to the Earth as they orbited the Sun. Based on astronomer Tacho Brahe’s observations of Mars’ elliptical orbit around the Sun, Kepler developed three laws that are still highly regarded today.
Kepler’s First Law: Planets follow elliptical orbits with the Sun at one focus.
Kepler’s Second Law: The closer a planet is to the Sun, the faster it moves.
Kepler’s Third Law: “The ratio of the squares of the period of time required for any two planets to complete an orbit of the Sun is proportional to the ratio of the cubes of their average distances from the Sun.”
His third law was very important. If one planet’s distance from the Sun was known, it was possible to determine the distance of any other planets from the Sun. Ultimately, we could then determine the size of the solar system.
To determine the solar system’s size, a group of astronomer’s among them Edmund Halley, proposed that Venus’s transit in front of the Sun could be used to determine the distance between Earth and the Sun. Although Venus’ orbit is slightly tilted compared to Earth’s orbit about the Sun, it can still be seen crossing in front of the Sun roughly every 100 years. In recording the path of Venus from two different locations on Earth, the parallax method could be used to find Earth’s distance from the Sun. Venus’ track would be displaced when viewed from different places on Earth, which can be compared to the Sun’s angular size. The distance between Venus and the Earth could be determined, and using Kepler’s third law, the distance between the Earth and the Sun could be determined. The distance from the Earth to the Sun is roughly 150 million kilometres.
Based on this knowledge, parallax methods could be used to determine distances beyond the solar system. Observations made from the surface of the Earth limit the distance to less than 10,000 kilometres, however, if the Earth’s motion through space is used, further distances can be determined. By allowing rotation of the Earth and recording observations from two different points during orbit, the viewing distance becomes millions of kilometres. The most effective observations are made at six months when the Earth has completed half of its orbit, corresponding to an observational distance of about 300 million kilometres.
The understanding and use of parallax is very important. In 1840, parallax was used by Friedrich Bessel to measure the distance to a star, which was 11 light years away. However, it was soon realized that far distances were much harder to measure because it was too difficult to measure angular positions in the sky. Distances outside of our galaxy could not be measured- the farthest within our galaxy was only 160 light years away.
Even though it cannot be accurately used to measure far distances, the parallax method can be used to construct a rough scale. Parallax is still important. In 1989, a Hipparcos satellite was launched to measure parallitic distances for over 100,000 stars. Over three years, catalogues of material were collected, and today it is still used as a primary reference and standard for determining astronomical distances.
Before light years were understood, observers, such as the Greeks, used parallax to estimate the distance to a planet or star. During the 17th century, Johannes Kepler constructed a scale model of the solar system by mapping positions of the planets relative to the Earth as they orbited the Sun. Based on astronomer Tacho Brahe’s observations of Mars’ elliptical orbit around the Sun, Kepler developed three laws that are still highly regarded today.
Kepler’s First Law: Planets follow elliptical orbits with the Sun at one focus.
Kepler’s Second Law: The closer a planet is to the Sun, the faster it moves.
Kepler’s Third Law: “The ratio of the squares of the period of time required for any two planets to complete an orbit of the Sun is proportional to the ratio of the cubes of their average distances from the Sun.”
His third law was very important. If one planet’s distance from the Sun was known, it was possible to determine the distance of any other planets from the Sun. Ultimately, we could then determine the size of the solar system.
To determine the solar system’s size, a group of astronomer’s among them Edmund Halley, proposed that Venus’s transit in front of the Sun could be used to determine the distance between Earth and the Sun. Although Venus’ orbit is slightly tilted compared to Earth’s orbit about the Sun, it can still be seen crossing in front of the Sun roughly every 100 years. In recording the path of Venus from two different locations on Earth, the parallax method could be used to find Earth’s distance from the Sun. Venus’ track would be displaced when viewed from different places on Earth, which can be compared to the Sun’s angular size. The distance between Venus and the Earth could be determined, and using Kepler’s third law, the distance between the Earth and the Sun could be determined. The distance from the Earth to the Sun is roughly 150 million kilometres.
Based on this knowledge, parallax methods could be used to determine distances beyond the solar system. Observations made from the surface of the Earth limit the distance to less than 10,000 kilometres, however, if the Earth’s motion through space is used, further distances can be determined. By allowing rotation of the Earth and recording observations from two different points during orbit, the viewing distance becomes millions of kilometres. The most effective observations are made at six months when the Earth has completed half of its orbit, corresponding to an observational distance of about 300 million kilometres.
The understanding and use of parallax is very important. In 1840, parallax was used by Friedrich Bessel to measure the distance to a star, which was 11 light years away. However, it was soon realized that far distances were much harder to measure because it was too difficult to measure angular positions in the sky. Distances outside of our galaxy could not be measured- the farthest within our galaxy was only 160 light years away.
Even though it cannot be accurately used to measure far distances, the parallax method can be used to construct a rough scale. Parallax is still important. In 1989, a Hipparcos satellite was launched to measure parallitic distances for over 100,000 stars. Over three years, catalogues of material were collected, and today it is still used as a primary reference and standard for determining astronomical distances.
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