What is a Planisphere?
A tool used in astronomy, a planisphere is a star chart that gives the exact date and time of a constellation.
Astronomers use this amazing tool for four reasons: it helps them identify constellations, discovers when constellations are in the sky and the time it came out, it finds the best time and date to see these constellations, and it also can locate a planet in the sky depending on which month.
Planispheres are amazing tools as well because of these four helpful characteristics: they can keep information about the stars and planispheres can be kept with you at all times, planisphere is immortal: they last forever, you don’t need knowledge on how to use them, and you also don’t need wi-fi, power, or technology to run a planisphere.
Now that you know what a planisphere is, you might be wondering how do use these amazing star discs? In this article, we will be going over how to use such a wonderful tool!
What to Look for When Getting a Planisphere?
Before you know how to use one, you want to know what to look for before getting one. The most essential component of a planisphere is the specific latitude.
Latitudes vary for planispheres because they only work for observing 10 degrees of the given latitude. The rest you see will be different from the chart, thus it makes it useless.
So, to ensure you find one that will work for you, you want to get one that suits your location. Here are the three best latitudes and the best locations for each:
- 20-30 Degrees N: Southern states of the US, including the Florida Keys
- 30-40 Degrees N: most of the US and Southern Europe
- 40-50 Degrees N: North America, Southern Canada, Europe, and the UK
Take this as an example. Maine is 45 North, so the best one, if you live in this state, is a planisphere that is 30-40 degrees. Still not sure what latitude your town is? Check it with Google! Type in the name of your town with latitude and find the one that says N (North).
Another important thing when before buying one is to find one that is made out of plastic rather than card/paper. Plastic designs tend to last more and can withstand heavy rain or dew, unlike the other option.
What Does A Planisphere Look Like? :
Once you find the right planisphere that fits with your latitude, you will see a few things with the planisphere. It is designed to have two big circles that are held with a pivot that is at the center. As you see on the back circle, it has a star map printed on it, while on the front disc has an oval-shaped window which lets you see parts of the sky depending on the time you choose to observe.
How to Use a Planisphere?
Once you get a planisphere, you now want to know how to use it. You can use the planisphere for multiple purposes. Here are the purposes and how to use a planisphere when doing these purposes :
1. Using the Planisphere to Learn Circumpolar Constellations : Learning constellations is an essential part of astronomy. There are constellations titled the circumpolar constellations. These constellations are ones that circle the pole.
The important thing when using the planisphere is that the center rivet is the Northern Celestial Pole which you can see the Polaris.
Surrounding the NCP are the circumpolar constellations. Such constellations like Ursa Major, Lynx, Perseus, and Cygnus are ones that are never hidden, thus you can locate these easily.
2. Using the Planisphere to Locate Other Constellations : Unlike the circumpolar constellations, other constellations in the sky usually appear in the night sky at different periods. An example of one of these far-distant constellations is Orion.
Since Orion is a winter constellation and not a circumpolar one, If you ever want to spot Orion, you’ll need to set the planisphere right. To do this, you will need to make sure the Orion constellation is the highest point while in the sky, which happens on the 15th of June.
If you look at the rear part of your planisphere, Orion comes out usually on these different dates and times: February 1st at 7 PM, January 15 at 2 AM, November 16 at 2 AM, or October 1st at 5 AM. With that in mind, you want to turn the front wheel halfway so the midday is aligned with the 14th of December and you cannot see Orion in the window.
3. Finding the Zenith : If you are using a planisphere, you might notice a small, blue x marked on it. What does it mean? The small x is the zenith, a given point or celestial sphere that you can detect in the sky.
The zenith marks the overhead point. The date and time of the zenith is on November 20th. Once you can see your zenith, you will get pointed in the right direction.
4. Using the Planisphere for Stargazing ; Outside of using the planisphere for looking for constellations, you can also use it for stargazing! If you want to use the planisphere for this method, here are the steps to follow if you want the best of stargazing:
- First, face South.
- Hold the planisphere in front of you.
- Rotate the planisphere so “South” is pointing to the horizon you are facing (which is South).
- Make note of the zenith (or overhead view) and compare the planisphere
- You want to compare the southern horizon view with the planisphere’s view.
- Now you want to go midway between the zenith and horizon to get a sense of the planispheres scale and reality.
- And there you have it! Now you just need to repeat these steps in different directions.
5. Using a Planisphere for Finding Planets : Did you know that you can also use this helpful tool for finding planets?
While not printed on your planisphere, you can still find the planets because of a special planisphere called the Firefly! You can find planets like Venus, Mars, Jupiter, and Saturn using this special type of planisphere. But how do you find these planets?
If you turn your planisphere over, you will see four charts of numbers, These numbers are also called “planetary numbers.” These planetary numbers include the name of the planet at the top including the years, months, and the numbers that line up with the year and month. Using one of these as an example, let us say the year is 2017 and its January and you want to find Mars.
Take the first two numbers (341 and 3), the 341 resembles the month of January and 3rd resembles the month of February. They are both measures of degrees on the outermost of the planisphere.
Using a straight edge, you can mark 341 to the central rivet. Now do the same with February by marking the 3 on the first day. Now that you have the two lines, you now can see the trail on which Mars will be traveling between these times.
6. Using a Planisphere to Find the Sunrise or Sunset : Want to know the time of the sunset or the sunrise?
A planisphere can help you with that! Align the planisphere on November 20th. You will need to imagine a line going to the center of the sphere to the 20th (it should look like the sun crosses the ecliptic.
You then want to move the front disc so that the Eastern horizon is touching the sun’s location on the ecliptic. It should match on the 20th of November.
Now repeat the same process, only your now facing West. Voila! you should be able to see the sunrise or sunset. You normally see the sunrise before 8 am (though this does vary depending on where you live) and the sunset is viewed around 4 pm.
You can use the planisphere for many things when it comes to finding constellations/planets/sunrise/sunset. That is what makes the planisphere a special tool for any new astronomer.
Other Things to Know About the Planisphere :
Now that you know how to use the planisphere, here are some other things that weren’t mentioned before.
Setting up a planisphere depends on what time and date you want to see a certain constellation. A thing to remember for planispheres is that each are different, but each one comes with instructions.
Here are some other amazing facts surrounding the planisphere that you might have not known:
- Planispheres were invented way back 150 BC by the Ancient Greeks. The planisphere at this time was known as the astrolabe, an invention created by Hipparchus who was an astronomer, geographer, and mathematician.
- NASA has its planispheres: they call them star wheels or the Kepler star wheels.
- Did you know that you can make your planisphere at home? If you don’t have the money to buy one, all you need to do is to print out two discs and then assemble them.
- Other astronomical instruments like the planisphere include the Antikythera and the Cosmolabe.
These are just some interesting facts about the Planisphere. So if you want to use this amazing tool to find constellations or other things in the dark, peaceful night sky, get a planisphere!
Written by : Alicia Barton
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