지구는 행성인가?
지구는 태양 주위를 공전하는 8개의 행성 중 하나입니다.
행성은 방랑자를 의미하는 그리스어 plantetes에서 유래 되었습니다.
역사를 통틀어 행성은 움직임에 따라 별과 구별 되었습니다.
별은 같은 위치에 있지만, 행성은 움직이는 것처럼 보였습니다. 행성은 고정되어 있지 않고 밤하늘을 떠도는 것처럼 보여, 방랑자 라고 불렀습니다.
대부분의 인류 역사에서는 지구가 우주 중심에 있다고 믿었 습니다. 태양, 달, 행성, 별이 지구를 돌고 있다고 믿었습니다. 지구를 행성으로 생각하지 않았습니다.
지구가 태양 주위를 돈다고 주장한 일부 그리스 철학자들이 있기는 했습니다.
https://youtu.be/pOqFosoXlss
1515년, 코페르니쿠스는 태양 중심설에 대해 처음 논문을 썻지만, 사망 직전인 1543년까지 그의 연구를 발표하지 않았습니다. 코페르니쿠스는 당시 자신의 생각이, 논란의 여지가 있다는 것을 알고 있었고, 자신의 생명과 명성을 위험에 빠뜨리기보다는, 자신의 업쩍을 출판하기 위해 밝히지 않았습니다.
태양 중심설은 지구는 하나의 행성이며, 다른 행성과 마찬가지로 태양 주위를 돈다는 것입니다.
몇 년 후인 1610년 천문학자 갈릴레오 갈릴레이는 역사상 최초로 망원경으로 천체를 관측하여, 지구를 포함한 모든 행성이 태양 주위를 돈다는 것을 증명하였습니다. 이러한 주장은 고착되었고 지구는 행성으로 알려지게 되었습니다.
과거에는 별과 관련하여 위치가 변하는 물체를 행성으로 정의 했지만 오늘날은 그렇지 않습니다. 행성을 단순히 움직이는 물체로 정의한다면, 다른 여러 유형인 천체, 달, 소행성, 혜성도 행성으로 정의되야 할 것입니다.
2006년 국제 천문학회에서 행성에 대한 천문학적 정의를 만들었습니다. 정의는 세 가지 기준을 충족해야 합니다.
첫째, 물체는 태양 주위를 공전해야 한다.
둘째, 물체는 자신을 구형으로 끌어당길 수 있는 충분한 중력이 있어야 한다.
셋째, 물체는 궤도를 도는 잔해물을 치울 수 있을 만큼 충분한 중력이 있어야 한다.
이 기준으로 볼때, 당시 행성이었던 명왕성이 왜행성으로 분류 되었습니다. 지구는 세 가지 기준을 충족하여 행성으로 정의 되었습니다.
PLANETS
What is a Planet?
Introduction
This seemingly simple question doesn't have a simple answer. Everyone knows that Earth, Mars and Jupiter are planets. But both Pluto and Ceres were once considered planets until new discoveries triggered scientific debate about how to best describe them—a vigorous debate that continues to this day. The most recent definition of a planet was adopted by the International Astronomical Union in 2006. It says a planet must do three things:
It must orbit a star (in our cosmic neighborhood, the Sun).
It must be big enough to have enough gravity to force it into a spherical shape.
It must be big enough that its gravity cleared away any other objects of a similar size near its orbit around the Sun.
Discussion—and debate—will continue as our view of the cosmos continues to expand.
The Scientific Process
The Scientific Process
Science is a dynamic process of questioning, hypothesizing, discovering, and changing previous ideas based on what is learned. Scientific ideas are developed through reasoning and tested against observations. Scientists assess and question each other's work in a critical process called peer review.
Our understanding about the universe and our place in it has changed over time. New information can cause us to rethink what we know and reevaluate how we classify objects in order to better understand them. New ideas and perspectives can come from questioning a theory or seeing where a classification breaks down.
An Evolving Definition
An Evolving Definition
Defining the term planet is important, because such definitions reflect our understanding of the origins, architecture, and evolution of our solar system. Over historical time, objects categorized as planets have changed. The ancient Greeks counted the Earth's Moon and Sun as planets along with Mercury, Venus, Mars, Jupiter, and Saturn. Earth was not considered a planet, but rather was thought to be the central object around which all the other celestial objects orbited. The first known model that placed the Sun at the center of the known universe with the Earth revolving around it was presented by Aristarchus of Samos in the third century BCE, but it was not generally accepted. It wasn't until the 16th century that the idea was revived by Nicolaus Copernicus.
By the 17th century, astronomers (aided by the invention of the telescope) realized that the Sun was the celestial object around which all the planets—including Earth—orbit, and that the moon is not a planet, but a satellite (moon) of Earth. Uranus was added as a planet in 1781 and Neptune was discovered in 1846.
Ceres was discovered between Mars and Jupiter in 1801 and originally classified as a planet. But as many more objects were subsequently found in the same region, it was realized that Ceres was the first of a class of similar objects that were eventually termed asteroids (star-like) or minor planets.
Pluto, discovered in 1930, was identified as the ninth planet. But Pluto is much smaller than Mercury and is even smaller than some of the planetary moons. It is unlike the terrestrial planets (Mercury, Venus, Earth, Mars), or the gas giants (Jupiter, Saturn), or the ice giants (Uranus, Neptune). Charon, its huge satellite, is nearly half the size of Pluto and shares Pluto's orbit. Though Pluto kept its planetary status through the 1980s, things began to change in the 1990s with some new discoveries.
Technical advances in telescopes led to better observations and improved detection of very small, very distant objects. In the early 1990s, astronomers began finding numerous icy worlds orbiting the Sun in a doughnut-shaped region called the Kuiper Belt beyond the orbit of Neptune—out in Pluto's realm. With the discovery of the Kuiper Belt and its thousands of icy bodies (known as Kuiper Belt Objects, or KBOs; also called transneptunians), it was proposed that it is more useful to think of Pluto as the biggest KBO instead of a planet.
The Planet Debate
The Planet Debate
Then, in 2005, a team of astronomers announced that they had found a tenth planet—it was a KBO similar in size to Pluto. People began to wonder what planethood really means. Just what is a planet, anyway? Suddenly the answer to that question didn't seem so self-evident, and, as it turns out, there are plenty of disagreements about it.
The International Astronomical Union (IAU), a worldwide organization of astronomers, took on the challenge of classifying the newly found KBO (later named Eris). In 2006, the IAU passed a resolution that defined planet and established a new category, dwarf planet. Eris, Ceres, Pluto, and two more recently discovered KBOs named Haumea and Makemake, are the dwarf planets recognized by the IAU. There may be another 100 dwarf planets in the solar system and hundreds more in and just outside the Kuiper Belt.
The New Definition of Planet
The New Definition of Planet
Here is the text of the IAU’s Resolution B5: Definition of a Planet in the Solar System:
Contemporary observations are changing our understanding of planetary systems, and it is important that our nomenclature for objects reflect our current understanding. This applies, in particular, to the designation "planets". The word "planet" originally described "wanderers" that were known only as moving lights in the sky. Recent discoveries lead us to create a new definition, which we can make using currently available scientific information.
The IAU therefore resolves that planets and other bodies, except satellites, in our Solar System be defined into three distinct categories in the following way:
A planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.
A "dwarf planet" is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.
All other objects,except satellites, orbiting the Sun shall be referred to collectively as "Small Solar System Bodies".