What Is a Planet?
A planet may be defined as a celestial body that circles a star or another stellar object and which fulfills three criteria, namely: the object must have enough size such that its gravity causes it to assume a circular shape; the body should also not create thermonuclear reactions because of its size; and lastly, the gravity of that body must be enough to push away any other bodies that are within its space.
There are currently eight planets in our solar system. Pluto is no longer classified as a planet. Instead, Pluto and other celestial bodies are now classified as dwarf planets because they do not fulfill all the criteria above. Another kind of planet is the double planet. By definition, double planets are celestial objects that orbit each other.
Planets Visible From Earth
There are five planets that can be seen with the unaided eye. These five planets are also the brightest, and they are Mercury, Venus, Mars, Jupiter, and Saturn. Typically an observer is able to spot each of these individual planets throughout the year. On rare occasions, however, all of the five planets appear at the same time.
The apparition of a planet refers to the period of time that a planet is visible. Mercury and Venus are well observed in the morning or the evening. For the case of the other planets, their apparitions begin when they are seen around dawn to the time they disappear during dusk. These apparitions vary among planets and may last anywhere between a couple of weeks to years depending on the time the planet takes to orbit the Sun.
The brightness of the planets to the eye is dependent upon the distance of the planet from the Sun. The reason for this is because planets do not produce their own light. Instead, they reflect the Sun’s light. Therefore, a planet reflects more light if it is closer to the Sun thus appearing brighter.
On rare occasions, all the five planets can be seen at the same time. However, the planets do not align themselves in a neat arc. Therefore, the person who wishes to see them needs to know what to look for and understand the differences between stars and planets. The most suitable time to view them would have been 45 minutes before the sunrise. Of the five, Mercury is usually the hardest to see, but it will be close to Venus. To the right of Venus, the other three planets are visible in a rough arc.
The last time such an event took place was in August 2016. Experienced people would have been able to see them even on January 20, 2016. Before January 2016, the five planets were all visible in January 2005 and in December 2004. The next time such an event will occur will be July 2020. Mercury will be even harder to spot in 2020 because of its anticipated closeness to the horizon. This phenomenon is rare and occurs a few times in a decade.