The sky ruler is a simple but effective device that will help you find objects in the sky and make measurements of their positions. Best of all, it’s easy to make from items hiding away in your kitchen drawer.
The sky ruler’s design is based on an ancient tool called a ‘cross staff’ that is thought to have originated around 400BC and probably evolved from instruments used by Arab navigators.
They used the cross staff to calculate their latitude, because it can measure the altitude of the Sun at noon.
Although the objects we see in the night sky are at varying distances from Earth, to us they appear to be fixed to the inside of an imaginary dark ball (called the celestial sphere), with our planet at the centre.
The stars seem stationary while the Sun, Moon and planets look like they move around us.
This illusion fuelled years of unfortunate debate between astronomers and the Church, but it does at least mean that we can describe the positions of celestial objects in terms of angles measured around the inside of this imaginary sphere.
You can use your sky ruler to determine the angles that separate objects in the sky, as well as their altitude above the horizon.
With this skill you’ll be able to improve the way you record your observations and describe them to other astronomers.
After all, ‘Mercury is quite low down over there’ is not as informative as ‘Mercury is 15º above the southwestern horizon.’
By setting the sky ruler to predetermined angles, it can also help you find objects described in star charts.
Or, if you have planetarium software on your computer, use the software to display the altitude and azimuth values (often shortened to alt and az) for an object, and then employ your sky ruler to find it in the night sky.
You can also use a ‘celestial positions calculator’ to get co-ordinates.
These values, which are measured in degrees, vary with time, so unless you have the computer by your telescope you’ll need to adjust the software to correspond with the time and date you plan to observe.
When scheduling your evening’s observations, make a note of the altitude and azimuth values for your targets at various times in case you change your plans.
What you need to get started
Hair bands and a lollipop stick
If you use plastic for the cross piece you’ll need to add a strip of wood (a lollipop stick) to the top. This provides a layer to stick the pins into. Use two hair bands, (the kind made with round elastic), to hold the cross-piece and main scale together. Alternatively you can use rubber bands.
For the sights you need to use four map pins, the kind with plastic beads on the top. White pins are easier to see at night, so consider painting the tops white if necessary.
You need a 30cm (12-inch) ruler for the main scale. This could be made of plastic or, alternatively, smooth wood. To make the cross piece you need a short piece of ruler 7cm (2.75 inches) long. Again, a similar strip of wood or plastic could be used.
A craft knife or scissors are needed to cut out the printed sky ruler scale, which you’ll find in the link at the top of this article. A junior hacksaw is useful to cut the cross piece to size. Use glue or double sided tape to stick down the printed scale and lollipop stick.
Get your bearings
The azimuth value tells you the direction to look in. Due north is 0º or 360º, 90º is east and so on.
A compass is obviously useful for this, although if you can see Polaris (found by following the ‘pointers’ of the Plough in Ursa Major), you can estimate where north is and work out the direction to look from there.
The altitude tells you how high to look in the sky; it’s measured from the horizon upwards. The horizon is 0º, while 90º is directly overhead (called the zenith).
The sky ruler measures angles ranging from 2.5º to 40º.
To locate your object you set the sky ruler to the altitude given by the computer and aim it in the azimuth direction.
If your object is too dim for the naked eye, you will at least know which area to scan with your binoculars or where to point your telescope’s finderscope.
While not quite pocket-sized, the sky ruler is easy to slip into a telescope case and it is surprisingly accurate.
However, if you want even more precision and enjoy the challenge of building a good quality instrument, why not scale things up?
The original cross staffs of old were about a metre (3.3ft) long and made from hardwood.
If you’d like to make a larger one, on this month’s coverdisc there is a table of dimensions for marking out the scale graduations on sky rulers of varying lengths.
You could make your customised sky ruler even better by choosing good-quality materials or developing nifty customised features such as illuminated pins.
You might also consider replacing the hair band with a suitable sliding joint – perhaps with a locking screw.
The standard sky ruler may not be a match for a Go-To telescope mount, but it is a great aid to finding your way around the night sky.
You will soon learn to estimate angles and describe what you are seeing properly.
If you are a member of a local astronomy society or you take groups out observing, you could make a set of sky rulers that can be handed round to help you point out where things are.
They are good fun to use, especially for children and complete beginners.
After printing out the sky ruler scale from the link at the top, carefully cut round the three sections and glue the first section to the flat side of a ruler so that the eye symbol is at the end.
Add the remaining part of the scale.
Cut a 7cm (2.75 inches) long cross piece from a ruler using a junior hacksaw.
If it is plastic, glue a lollipop stick to the upper curved surface.
Stick the cross piece’s paper scale on top using glue.
Covering the scales with clear tape makes them more durable.
Push the four map pins into the positions marked on the cross piece.
Use the hair band to hold the two parts together as shown.
The cross piece should be secure, but easy to slide, so you may need to experiment with different-sized bands.
To measure an angle, hold the sky ruler up to your eye with the end touching your cheekbone.
Now move the cross slider so a pair of pins line up with the angle you are measuring.
Try working out the length of the Plough in degrees in the constellation Ursa Major.
Note where the cross piece lies on the scale.
If you used the outer pins (A and D) you can read off the angle directly.
If you used pins C and D, divide the reading by two; for B and C, divide by four.
This part of The Plough is 10º across.
If you know the altitude and azimuth of an elusive object, set the cross piece to the desired altitude and, with the cross piece held vertically, aim in the azimuth direction.
If one pin lies on the horizon, the corresponding pin should be close to your target.
This article first appeared in the August 2008 issue of Sky at Night Magazine