Editing 2596: Galaxies
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This is another comic with a [[:Category:Facts|Fact]], the second in a row of these fact comics to use an Astronomy fact. | This is another comic with a [[:Category:Facts|Fact]], the second in a row of these fact comics to use an Astronomy fact. | ||
| − | Our best approximation of the number of {{w|galaxies}} in the {{w|observable universe}} is about 200 billion (2 × 10<sup>11</sup>). That's a lot of galaxies, | + | Our best approximation of the number of {{w|galaxies}} in the {{w|observable universe}} is about 200 billion (2 × 10<sup>11</sup>). That's a lot of galaxies, and here [[Randall]] exemplifies this by showing a small circle and estimating that when the comic's picture is viewed at a typical arm's length, expanded to full screen on your typical smartphone, the circle contains roughly 50,000 galaxies (that means of course not the small circle itself, but the volume defined by the viewer's eye, that circle, and an onward conical extension into deep space — and simultaneously back in time — to the respective limits of the observable universe). Most of those far-away galaxies are undetectable by even our most powerful astronomical instruments today, and comparatively few could be seen (let alone positively identified as such) by the naked eye. For example, in the {{w|Hubble Deep Field}}, an image of a small region in the constellation Ursa Major, about 3,000 visible galaxies can be identified. |
Measuring in the mid-point of the lines, the circle is about one fortieth of the width of the frame of the comic. The absolute circle size depends on the display resolution, size and mode, but it can reasonably be taken to be 1mm diameter, or 0.5mm radius, giving a total area π r<sup>2</sup> or about π/4 square millimeters. You're probably holding the phone about a half a meter away from your eye. The surface area of a sphere is 4 π r<sup>2</sup>. With a radius of one-half meter, that comes out to be π square meters. Thus, the area of the circle is about 1/4000000 of the area of the sphere, 200 billion galaxies divided by 4 million is the 50,000 average mentioned in the cartoon. A similar mathematics was used for the comic [[1276: Angular Size]], in which the projective sphere was at the Earth's own radius and cross-sectional areas of objects were compared, rather than an approximate count of objects within a given angular spread. | Measuring in the mid-point of the lines, the circle is about one fortieth of the width of the frame of the comic. The absolute circle size depends on the display resolution, size and mode, but it can reasonably be taken to be 1mm diameter, or 0.5mm radius, giving a total area π r<sup>2</sup> or about π/4 square millimeters. You're probably holding the phone about a half a meter away from your eye. The surface area of a sphere is 4 π r<sup>2</sup>. With a radius of one-half meter, that comes out to be π square meters. Thus, the area of the circle is about 1/4000000 of the area of the sphere, 200 billion galaxies divided by 4 million is the 50,000 average mentioned in the cartoon. A similar mathematics was used for the comic [[1276: Angular Size]], in which the projective sphere was at the Earth's own radius and cross-sectional areas of objects were compared, rather than an approximate count of objects within a given angular spread. | ||
