Hi I’m David Fuller from the “Eyes on the Sky” video series.
In this last Stargazing Basics video, we’ll learn how to easily measure distance in the
sky, so you can find constellations or objects more easily either naked eye, with binoculars
or telescopes.
In our first video, we learned that a line called the meridian splits the sky into equal
halves, from north to south.
If we were to place a giant protractor in place of that line, it would appear as if
the sky was 180 degrees from horizon to horizon.
Although space is actually infinite, our eyes make the night sky appear like a “half sphere,”
so for all practical purposes, it’s easier to think of the sphere.
And though a sphere should technically be measure in radians, degrees is a concept people
understand more readily and works for our purposes.
So horizon to horizon is 180 degrees – that’s easy enough.
And if we measured another large distance, from horizon to zenith, that would produce
a right angle, or 90 degrees.
Still pretty simple, right?
But to measure smaller angles than that, we need a measuring tool.
A ruler doesn’t work, because that would be for linear measurement, and holding a protractor
to our eyes is a bit impractical.
So what to do?
Easy: Use your hands!
Check this out: Hold out your hand at arm’s length.
Now spread your thumb and pinky as far away from each other as possible.
If you look across your thumb and pinky, that distance is approximately 25 degrees.
Don’t worry – this works for almost everyone.
Don’t believe me?
Look for the Big Dipper in the night sky.
See the last two stars in the “Bowl” of the dipper?
Draw a line through them, from the “bottom” of the bowl towards the top.
Now hold your hand at that top star, along that line.
The other side of your outstretched hand should be near Polaris, the north star, because that
distance is NEARLY 25 DEGREES.
But sometimes we need to measure smaller distances than 25 degrees.
This time, hold up your forefinger and pinky, and stretch them out.
The distance across them is about 15 degrees.
This is about the distance from Orion’s belt to the star Aldebaran – this way – or
the star Sirius, going this way.
You can find these stars in the winter sky.
For another smaller tool, hold up your fist.
Across the top of your fist from side to side is about 10 degrees of sky.
We can ‘calibrate” that by looking at FIND A GOOD CIRCUMPOLAR CALIBRATION.
Two split that in half, now hold up these three fingers – this approximates 5 degrees
of sky.
Those two stars at the end of the Big Dipper are about 5 degrees from each other.
Hold up your hand and see if your fingers match that.
And lastly, the one degree tool.
Simply hold up your pinky!
This one amazes many people, because the actual angular distance across the Full Moon is only
half of a degree.
So holding your pinky at arm’s length, you can cover the WHOLE Moon!
Of course, mixing and matching these can help you find even more – two heands like this
can measure halfway across the 90 degrees of horizon to zenith, approximating 45 or
50 degrees or so.
Use other combinations to create 30, 35 or 40 degrees, just by using two hands.
But how will you know how far an angle is in the sky from a star chart?
The declination lines will tell you degrees, but only in that direction.
Try downloading the “Skymaps” all sky star charts each month.
These charts are about 180 millimeters across.
Though not terribly useful at the edges, as the sky diagrams are “stretched” there,
you can use a simple ruler with millimeters on it to measure approximate angular distances
in the sky, helping you “hop” from bright stars or well known constellations, to dimmer
ones.
Give it a try – it’s really easy, and works for just about everyone.
Thanks for watching; I’m David Fuller.
Keep your eyes on the sky and your outdoor lights aimed down by using dark sky friendly
lighting fixtures, so we can all see, what’s up.