Note: I got much of the information on moon cycles from http://www.hollyi.com/articlesNart/01sunmoon.htm. If you want more in-depth info about moons, rainbows, and what “west” really means, I highly recommend checking it out. Click on her name at the top for even more world-building goodness.
There are many styles of planet, and while you are welcome to create any planet you desire, residents of most planets have come to expect a few regularities. If you seek to create a believable and recognizable planet, please keep these tips in mind. As the most common reference point, we will use the planet “Earth”.
A) Give your planet a sun.
For example, earth orbits a star named 'Sol'. Because it orbits this star, the star is called a sun. If your planet has the Earth-normal axial tilt of about 23.5 degrees, they will experience summer when their side of the planet is tilted toward the sun, and winter when it is tilted away.
What that means to your planet will vary; it may be in an ice age, for example, and even summer could be unbearably cold. Just remember that there is still a summer. The days are longer and warmer. Life will occur more actively during this time than in the winter months. (Quick thought: What would happen if bears had to hibernate twice as long?)
B) Give your planet a moon.
For example, earth has a moon named 'Luna'. Luna has a number of interesting functions. She's responsible for regulating tides – the ebb and flow of the ocean that affects life and, occasionally, history. Keep in mind that the highest high tides and the lowest low tides occur during the full moon, when Luna is at her roundest, and the New Moon, when shadow covers her. Tides are weakest at the equator. Assuming a planetary tilt of ~23 degrees, tides will be weakest at the equator and strongest right about at 23 degrees north or south of the equator. (As a reference, the north and south equators are at 90 degrees.)
A further note, if you use Luna and Earth as your template: During the equinoxes, a new (dark) moon rises at sunrise. Halfway between the New Moon and Full Moon, heading towards full it will rise at the actual middle of the day; heading towards new, it will rise in the dead center of the night. The moonrise will stay pretty close to that near the equator. Closer to the poles, it will still maintain those times during equinoxes, but the rising times will be a few hours earlier in the middle of summer and a few hours later in the middle of winter.
Of course, if your planet has no moon, there will still be weak tides; about a third of earth normal, assuming a sun like Sol. If your planet has two or more moons, the simplest thing to do is to add the tidal effects together; they each have their separate patterns that combine into one megapattern. As a rough estimate of the amount of effect of the moon: If it looks bigger than Luna does to the inhabitants of Earth, it will have a greater effect on the tides. If it looks smaller than Luna looks on Earth, it will have a smaller effect on the tides. It's not exact, but how exact do you need it?
The tides will affect the life cycles of your planet; sea turtles lay their eggs at high tide, so that the eggs won't get washed out to sea. They hatch just a few weeks later, again at high tide.
Two more types of moons I'd like to mention. They are the moons of Earth's brother, Mars. Mars' smaller moon, Phobos, orbits its planet quicker than Mars turns. As a result, it seems speed backwards across the sky. It rises in the west, zips across the sky in four hours, and returns 11 hours later. It cannot be seen further north or south than 70 degrees. It takes just under a third of a day to complete a cycle from one full moon to the next. (For those counting, that means it will complete half its phases while you watch it in the sky.)
Deimos, Mars' smaller moon, is higher in the Martian sky. It can be seen as far north and south as ~83 degrees. It is pale for a moon, though brighter than most stars, and looks to be less than 1/10th earth's Luna. It is in the sky for two and two-thirds days. While it's up, you can see it go through it's phases in a little more than a day and a quarter. (Phobos and Deimos days are measured using Martian days.) (What effect would using one of these moons have on the inhabitants of your world? Consider carefully.)
C) Give your planet plates.
I'm not talking about special holiday plates. (Although it has its eye on that set it saw in the store window. You know which ones I'm talking about.) I'm talking about tectonic plates. Under their surface, planets are roiling masses of molten material. Many planets have epic volcanic activity to release the buildup of heat; this may be a more toxic and dangerous planet than one with tectonic plates, as Earth has.
Earth's surface is split into many different hunks of rock floating on its magma. As they crash into each other, some will be pushed under the earth's surface (cooling the molten bits, like dropping frozen stones into hot tea.). Other plates will get pushed up – creating mountains where they collide with other. And, since the plates are moving mostly on the east-west line because of the spinning earth, the mountains will mostly run north and south. (What if your planet doesn't have tectonics? Would it still have mountains? If yes,how do they form?)
That covers some Crash Course basics about designing planets. I'm hoping that even those not as interested in worldbuilding as I am will have found something interesting in today's topic. If not – tune in Saturday for a story!
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