Angular Size of the Big Dipper

Angular distance

Angular distance
Angular distance

Angular Sizes and Distances

In astronomy, the size and distances between objects in the sky is given as a measure of their angular diameter as seen from Earth. These distances are measured in degrees and radians. The angular diameters of these objects are usually very small; therefore, it is very common to represent them as degrees, arcminutes, arcseconds.

A circumference is equal to 360°; one degree is 1/360; one arcminute is 1/60 of a degree; and one arcsecond is 1/3600 of one degree. To put this in perspective, the Moon has an angular size of 1/2 of a degree or 30 arcminutes which is the same as 1,800 arcseconds. The largest lunar craters have angular sizes of 2 arcminutes across.

Using Your Hand to Measure Angular Distances

Angular Distance
Angular Distance

Covering Two Moons With your Thumb Finger

Hipparcos satellite, launched into orbit by the European Space Agency in 1989, measured large and small angles of 118,218 stars within 20 to 30 milliarcseconds which are very small angles; however, to measure angles greater than 1/2 of a degree, you can use your own hands.

If you hold your hand at arm´s length; the angular distance that you can measure with your thumb finger is one degree. With that finger you could cover two moons. Your three middle fingers cover the distance of 5°; With your fist closed, you can measure 10° in the sky; and the angular distance from the tip of your index finger to the tip of your pinky finger is 15°.


Angular Size of the Big Dipper

Big Dipper
Big Dipper

The Big Dipper

There are many celestial objects that you can use to practice measuring angular sizes. One of this objects is the Big Dipper. The Big Dipper is a circumpolar (never sets due to its proximity to the celestial pole) asterism (group of stars) which can be seen throughout the year.

Using your hand, you´ll see that the Big Dipper measures approximately 25° across form one end of the bowl to the tip of the handle. With the skills you´ve learned, you can look for more celestial bodies and measure their angular size or compare angular distances from other night sky objects.

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