Students often struggle to grasp the vast distances that the study of astronomy demands. Obviously, we cannot travel these distances, so direct experience is limited. We must peer out at the universe through our eyes (or substitute technology). This adds a further complication, because we do not see linear sizes—we perceive angular sizes—but our brains get so good at translating these perceptions into linear interpretations that students often struggle to recognize and interpret the meaning of angular sizes. I present here a sequence of simple activities that can get students more comfortable in working with angular size measurements, starting with simply holding up their pinky finger at arm’s length. Step by step, the students can extend the angular size of their finger out into their environment, to measure nearby objects, the Moon, the Sun, and even distant galaxies.

1.
Michael
Zeillik
, “
An astronomy angulatorTM
,”
Phys. Teach.
39
,
187
188
(
2001
).
2.
Michael
LoProsto
, “
How big is the Moon?
Phys. Teach.
38
,
179
180
(
2000
) describes simple experiments modeled on ancient techniques to estimate these quantities, but for the purposes of this article it is sufficient to look them up.
3.
Helen
Ross
and
Cornelis
Plug
,
The Mystery of The Moon Illusion—Exploring Size Perception
(
Oxford University Press
,
2002
).
4.
See also
Mark
Moldwin
, “
How big is our Sun?
Phys. Teach.
38
,
115
116
(
2000
).
5.
Benjamin J.
Shappee
and
K. Z.
Stanek
, “
A new cepheid distance to the giant spiral M101 based on image subtraction of Hubble Space Telescope/Advanced Camera for Surveys observations
,”
Astrophys. J.
733
,
124
187
(
2011
).
6.
Michael
LoPresto
, “
A classroom activity and laboratory on astronomical scale
,”
Phys. Teach.
55
,
442
443
(
2017
) offers suggestions for other objects in space with useful size–distance ratios.
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