Telescope Reviews: Angular Separation Ruler

Hello CN! I have come back after a couple years of sparse observing time spent with my beloved Z10 with that nostalgia factor of Astronomy hitting hard again. I have come across a neat tool in an astronomy lab at school that can be used for amateur observing, particularly keeping records of angular distances.

In this lab I am tracking planetary motion by estimating the angular separation of Mars from the nearby stars Regulus, Algieba, and Denebola of Leo and then estimating where mars is on a starmap with axes in right ascension and declination. The plan is to observe and calculate Mars' motion on the celestial sphere over a couple months and then characterize the properties of its orbit and see if the planet is slowing down of speeding up in orbit, etc.

In this instance angular separation is in the familiar unit of degrees. There are 90 degrees of separation from horizon to zenith and another 90 from zenith to the opposing horizon. It is often an interest to know the angular separation of two celestial objects projected onto the night sky. I have read of tricks to do this approximately with your human limbs and digits, 1 degree for a pinky at arms length, 25 degrees for a hang low sign at arms length, etc.

This particular tool makes use of 3 items:
1.)A foot ruler with metric units (centimeters)
2.)A piece of string that doesn't stretch too much
3.)Some tape

You will need to first use a measuring device to measure a length of string to about 114.6 cm. I found measuring tape to work so long as the string is stretched into a line along the measuring tape. Next tape one end of the string to one end of the ruler and the other end to the other end of the ruler. Do this in a way that the length of string available outside of the tape is approximately the same as your measured, i.e. don't place too much string under the restraining tape. Your device is now ready!

First hold the ruler in one extended hand with the centimeter marks facing toward you and use your free hand to hold the string back towards you like you are making a bow for archery. The strings should approximately form an isosceles triangle. Now hold the vertex of the triangle on your finger next to your eye. Each centimeter on the ruler corresponds to about 1 degree of angular separation in the sky. To use this device properly you will need to brace yourself and your arm onto something stable so as to not move too much while measuring a distance. You will also need a source of light on the ruler to see, preferably a red flashlight.

Hopefully some of you will find this tool useful when stargazing to keep track of the wanderers in the night sky, the planets.

Clear dark skies!