Comparing Apparent Sizes of Celestial Objects

Last week I had the opportunity to talk to a group of 5th grade students about the night sky. It was really energizing to see the interest these kids had in the topic. They asked a lot of questions, including one that I get asked almost every time I post or show a picture to friends and family. “How big is that?”

Honestly I love this question because it is a great launching point to so many ideas associated with astronomy and astrophotography (and a lot of other topics as well). As an astrophotographer, I think about what size object will fit on my camera sensor, just as a regular photographer thinks about what will fit in the frame of their camera. Just as with “regular” photography this is a function of the sensor size and the lens that is used, or for me the camera and telescope combination. Since I have several combinations possible, it is helpful to have a size scale that I can use to compare to the Field of View (FOV) of my camera/scope set-ups.

For astrophotography the normal size scale used is the angular size; measured in degrees, arcminutes, and arcseconds. This scale measures how many degrees (or more often fractions of a degree) of the sky the object covers. This scale measures the apparent size of the object, that is the size it appears to a viewer on Earth. From the horizon to the zenith (directly overhead) is 90°. For perspective on this scale the full moon is ~0.5° and we can hypothetically stack 180 full moons from the horizon to the zenith.

Many objects in the sky are much smaller than the moon so we also use fractions of degrees measured in arcminutes (symbol ′) and arcseconds (symbol ″). One degree (1°) is made up of 60 arcminutes (60′) and an arcminute is made up of 60 arcseconds (60″). A human with 20-20 vision can resolve objects down to ~1′. (As a side note, we can thank the ancient Sumerians and Babylonian astronomers for these units.)

The angular size scale used for apparent size of objects takes into account the fact that objects at large distances appear small compared to objects that are closer to the observer. Most people are familiar with the fact that the sun and moon appear to be the same size to a viewer on Earth, while knowing that the sun is much bigger than the moon. The similarity in apparent size of the sun and moon is why we get such interesting solar eclipses on Earth.

The table below has the apparent and actual size of several astro targets. You can see that size and distance lead to apparent sizes that make sense. Below the table are a few specific pictures I have taken that show some of the objects in the table. (A final note, the solar system objects below are shown in kilometers (km) while the extra solar objects are shown in light years (ly). A ly is equal to ~9.5 trillion km).

Next time I’ll talk about the telescopes and camera sizes used for these pictures and how their characteristics help me match targets to telescopes and cameras.

Till next time, clear skies,

Paul

Whirlpool Galaxy (M 51)

Jupiter

Pleiades Star Cluster (M 45)

The Moon and Sun (Total Solar Eclipse)

Fireworks Galaxy (NGC 6946)