Editing 1276: Angular Size

{{comic
| number    = 1276
| date      = October 11, 2013
| title     = Angular Size
| image     = angular size.png
| titletext = If the celestial sphere were mapped to the Earth's surface, astronomy would get a LOT easier; you'd just need a magnifying glass.
}}

==Explanation==
This comic is a comparison of the {{w|angular diameter|angular diameters}} (or apparent diameter) of various celestial objects at the surface of the earth relative to a vertex at the center of the Earth as diagrammed in the opening panel. The objects' scales are compared to actual objects on earth. Each size given is for the object at its closest approach to earth.

London's {{w|M25 motorway}} is around 60 km (35 miles) across, a {{w|soccer field}} is about 100 meters long, a {{w|Table tennis table|ping pong table}} is 274 centimeters long, a {{w|laptop}} is about 35 centimeters across, the {{w|tilde}} symbol on a keyboard is about 5 millimeters long, and a cell of ''{{w|Escherichia coli|E. coli}}'' is about 2 microns long.

A simple {{w|Intercept theorem|formula}} can be used to find the size on earth of a celestial object when the size of or distance to the object is known. This is done by taking the radius of the earth, multiplying by the diameter of the object, and dividing by the distance to the object from the center of the earth.

The space objects referenced in the panels are:

*The {{w|Sun}} and the {{w|Moon}}, and also the open cluster {{w|Messier 25}}, have approximately the same size (around 0.5 degrees of arc) when seen from the Earth.

*{{w|Mercury (planet)|Mercury}}, {{w|Venus}}, {{w|Mars}}, {{w|Jupiter}}, {{w|Saturn}}, {{w|Uranus}}, and {{w|Neptune}} are the other planets of the {{w|Solar System}}.

*{{w|Io (moon)|Io}}, {{w|Europa (moon)|Europa}}, {{w|Ganymede (moon)|Ganymede}}, and {{w|Callisto (moon)|Callisto}} are the main moons of Jupiter; {{w|Titan (moon)|Titan}} is the largest moon of Saturn; and {{w|Triton (moon)|Triton}} is the largest moon of Neptune. {{w|Ceres (dwarf planet)|Ceres}} and {{w|Pluto}} are {{w|dwarf planet}}s.

*{{w|Phobos (moon)|Phobos}} and {{w|Deimos (moon)|Deimos}} are the moons of Mars. {{w|Eris (dwarf planet)|Eris}} is another dwarf planet. {{w|R Doradus}} and {{w|Betelgeuse}} are giant stars, respectively around 180 and 640 light-years away. R Doradus is the star with the largest apparent diameter (other than the sun, of course).

*{{w|4942 Munroe}} is an asteroid [http://blog.xkcd.com/2013/09/30/asteroid-4942-munroe/ named] after xkcd author [[Randall]] Munroe. {{w|Proxima Centauri}}, {{W|Alpha Centauri|Alpha Centauri AB}}, {{w|Barnard's star}} and {{w|Sirius}} are nearby stars (all within 10 light-years from the Sun).

*{{w|HD 189733 b}}, {{w|Gliese 581 g}}, {{w|Gliese 667 Cc}}, {{w|82 G. Eridani#Planetary_system|HD 20794 c}}, {{w|Tau Ceti#Planets|Tau Ceti c}}, and {{w|KOI-1686.01}} are {{w|extrasolar planet}}s; the parenthetical names are references to the comic [[1253: Exoplanet Names]]. However, some of the planets' parenthetical names do not match the table in the previous comic. For example, HD 20794 c is called "Legoland" rather than "Moonchild" in [[1253: Exoplanet Names]]. The {{w|black hole}} at the center of our Galaxy is {{w|Sagittarius A*}}, a massive object containing a mass more than 4 million times of our Sun.

*{{w|Voyager 1}} and {{w|Voyager 2|2}} are space probes launched in 1977, and currently around 125 and 100 {{w|astronomical unit}}s away, respectively.

The title text states that astronomy would be much easier if the celestial sphere were mapped to the earth - like a giant {{w|globe}}. Due to the distance of the stars you would just need magnifying glass to see the areas representing distant stars instead of an expensive powerful telescope to see huge distances.

Approximate values for the mappings to the Earth sphere (based on mean Earth radius at 6,371.0&nbsp;km):
{| class="wikitable sortable"
|-
! Panel
! Object
! Distance
! Diameter
! Size at Earth sphere
|-
| rowspan="4"| Panel 1
|| Sun
|| 149,600,000 km
|| 1,392,684 km
|| 59.3 km
|-
|| Moon <small>Semi-major axis</small>
|| 384,399 km
|| 3,476.28 km
|| 57.6 km
|-
|| Moon <small>Perigee</small>
|| 363,295 km
|| 3,476.28 km
|| 60.9 km
|-
|| Messier 25
|| 2,100 light-years <br/> 19.867 × 10^15 km
|| 19 light-years <br/> 179.753 × 10^12 km
|| 57.6 km
|-
| rowspan="7"| Panel 2
|| Mercury <small>closest approach</small>
|| 92 million km
|| 4,879.4 km
|| 0.34 km
|-
|| Venus <small>closest approach</small>
|| 41 million km
|| 12.103.6 km
|| 1.9 km
|-
|| Mars <small>closest approach</small>
|| 56 million km
|| 6,792.4 km
|| 0.77 km
|-
|| Jupiter
|| 778,547,200 km
|| 139,822 km
|| 1.14 km
|-
|| Saturn
|| 1,433,449,370 km
|| 120,536 km
|| 0.54 km
|-
|| Uranus
|| 2,876,679,082 km
|| 51,118 km
|| 0.11 km
|-
|| Neptune
|| 4,503,443,661 km
|| 49,528 km
|| 0.07 km
|-
| rowspan="8"| Panel 3
|| Io
|| 778,547,200 km
|| 3,643 km
|| 29.8 m
|-
|| Europa
|| 778,547,200 km
|| 3,122 km
|| 25.5 m
|-
|| Ganymede
|| 778,547,200 km
|| 5,262 km
|| 43.1 m
|-
|| Callisto
|| 778,547,200 km
|| 4,821 km
|| 39.5 m
|-
|| Titan
|| 1,433,449,370 km
|| 5,150 km
|| 22.9
|-
|| Triton
|| 4,503,443,661 km
|| 2,705.2 km
|| 3.8 m
|-
|| Ceres <small>Perihelion</small>
|| 380,995,855 km
|| 974.6 km
|| 16.3 m
|-
|| Pluto <small>Perihelion</small>
|| 4,437,000,000 km
|| 2,306 km
|| 3.3 m
|-
| rowspan="6"| Panel 4
|| Phobos
|| 56 million km
|| 26.8 × 22.4 × 18.4 km
|| 3.05 m
|-
|| Deimos
|| 56 million km
|| 15 × 12.2 × 10.4 km
|| 1.71 m
|-
|| Eris <small>Perihelion</small>
|| 5.723 × 10^9 km
|| 2326 km
|| 2.59 m
|-
|| Eris <small>Aphelion</small>
|| 14.602 × 10^9 km
|| 2326 km
|| 1.01 m
|-
|| Betelguse
|| 643 ± 146 light-years <br/>max. 7.464 × 10^15 km
|| 950–1,200 solar radii <br/>max. 1.671 × 10^9 km
|| 1.43 m
|-
|| R Doradus
|| 178 ± 10 light-years <br/>max. 1.778 × 10^15 km
|| 370 ± 50 solar radii <br/>max. 0.515 × 10^9 km
|| 1.85 m
|-
| rowspan="6"| Panel 5
|| 4942 Munroe
|| 2.2 AU (1.2 AU closest to earth, 179.4 × 10^6 km)
|| 9–10 km
|| 35.5 cm
|-
|| Proxima Centauri
|| 4.243 light-years <br/> 40.142 × 10^12 km
|| 0.141 solar radii <br/> 0.196 × 10^6 km
|| 3.11 cm
|-
|| Barnard's Star
|| 5.980 light-years <br/> 56.574 × 10^12 km
|| 0.196 solar radii <br/> 0.272 × 10^6 km
|| 3.06 cm
|-
|| Alpha Centauri B
|| 4.366 light-years <br/> 41.305 × 10^12 km
|| 0.865 solar radii <br/> 1.204 × 10^6 km
|| 18.6 cm
|-
|| Sirius
|| 8.6 light-years <br/> 81.362 × 10^12 km
|| 1.711 solar radii <br/> 2.382 × 10^6 km
|| 18.7 cm
|-
|| Alpha Centauri A
|| 4.366 light-years <br/> 41.305 × 10^12 km
|| 1.227 solar radii <br/> 1.708 × 10^6 km
|| 26.3 cm
|-
| rowspan="9"| Panel 6
|| HD 189733 b
|| 63 light-years <br/> 596.024 × 10^12 km
|| 1.138 Jupiter radii <br/> 159,117 km
|| 1.7 mm
|-
|| Gliese 581 g
|| 20.3 light-years <br/> 192.052 × 10^12 km
|| 1.3 to 2.0 Earth radii <br/> max. 25,484 km
|| 0.85 mm
|-
|| Gliese 581 (Parent star)
|| 20.3 light-years <br/> 192.052 × 10^12 km
|| 0.29 solar radii <br/> 403,878 km
|| 1.34 cm
|-
|| Black Hole at the center of our Galaxy
|| 25,900 light-years <br/> 245.032 × 10^15 km
|| Mass 4.31 × 10^6:<br/>12.684 × 10^6 km
|| 0.33 mm
|-
|| Gliese 667 Cc
|| 22.7 light-years <br/> 214.757 × 10^12 km
|| 2.0 Earth radii <br/> 25,484 km
|| 0.76 mm
|-
|| Gliese 667 (Parent star)
|| 22.7 light-years <br/> 214.757 × 10^12 km
|| 0.42 solar radii <br/> 584,927 km
|| 1.74 cm
|-
|| HD 20794 c (Parent star)
|| 20 light-years <br/> 189.214 × 10^12 km
|| 0.92 solar radii <br/> 1,281,269 km
|| 4.31 cm
|-
|| Tau Ceti C (Parent star)
|| 11.905 light-years <br/> 112.629 × 10^12 km
|| 0.793 solar radii <br/> 1,104,398 km
|| 6.25 cm
|-
|| Koi-1686.01 (Parent star)
|| 1033.8 light-years <br/> 9.780 × 10^15 km
|| 0.52 solar radii <br/> 724,195 km
|| 0.47 mm
|-
| Panel 7
|| Voyager probes
|| (Voyager 1 at 126.10 AU) 1.886×10^10 km
|| 20 meters (with antennas)
|| 6.76 µm
|}

==Transcript==
:The size of the part of Earth's surface directly under various space objects
:[Several images are shown, of space objects of differing size and at different distances from Earth, illustrating the differing sizes of their "shadows" as mapped onto Earth's surface viewed from the center of the Earth.]

:[The first image merely sets the stage: Earth is a full circle, with two figures — Cueball and Megan — standing on it; a small space object casts a "shadow" on Megan, while a slightly larger object, though proportionally farther away, casts just about the same size shadow next to Cueball.]

:[The second image is a map showing London, with the Thames running through it, and a ring highway running around it labeled "London's M25 Orbital Freeway". A grey circle is overlaid, just about the diameter of the M25; it is labeled "The Sun and The Moon (about the same size)".]

:[The third image has a small grey rectangle in one corner labeled "Soccer field" for comparison. The image is dominated by four large, grey circles, considerably larger than the soccer field, labeled "Saturn", "Mars", "Jupiter", and "Venus", with Mars slightly larger than Saturn, Jupiter approximately twice the diameter of Saturn, and Venus approximately three times the diameter of Saturn. Smaller circles are labeled "Mercury", "Uranus", and "Neptune", with Mercury still somewhat larger than the soccer field, Uranus about its size, and Neptune slightly smaller.]

:[The fourth image has the soccer field blown up to take up much of the view; its center circle, goal areas, and corner kick areas are visible. Labeled grey circles of various circles are again overlain: Callisto and Ganymede are about as large as one half of the field; Io, Titan, and Europa are somewhat smaller than half the field; and Ceres, Triton, and Pluto are much smaller (all three together would probably fit into the soccer field's center circle).]

:[In the fifth image, there is a different grey rectangle, this one labeled "Ping Pong table", with a few balls and paddles visible. An irregular ovoid labeled "Phobos" is about the size of the table, as is a circle labeled "R. Doradus". An irregular circular shape labeled "Deimos" is about the size of half the table; a circle labeled "Betelgeuse" is a little smaller, and a circle labeled "Eris" is a little smaller, though still comfortably filling most of half of the table.]

:[In the sixth image, a light grey image of laptop computer keyboard and screen is shown, viewed from directly above. An irregular shape labeled "4942 Munroe" is slightly larger than the laptop, while circles labeled "Alpha Centauri A", "Sirius", and "Alpha Centauri B" form a descending series somewhat smaller than it. Circles labeled "Proxima Centauri" and "Barnard's Star" are considerably smaller than the laptop: Proxima Centauri would fit on the trackpad, while Barnard's Star covers perhaps four keys on the keyboard.]

:[In the seventh image we see a greatly zoomed-in shape which is identified as the "Tilde on laptop keyboard". A circle labeled "HD 189733 b (permadeath)" is almost as large as the tilde is wide; a circle labeled "Tau Ceti C (giant dog planet)" is somewhat smaller. Circles labeled "Gleise 581 g (jelly-filled planet)", "Gleise 667 C c (PILF)", and "HD 20794 c (moonchild)" are all 1/2 to 1/3 as wide as the tilde is wide. A smaller circle labeled "Event horizon of the black hole at the center of our galaxy" fits comfortably within the tilde's stroke width. A very small dot off in one corner (much smaller than the tilde or anything else in the image) is labeled "KOI-1686.01 (emergency backup earth)".]

:[Finally, in the eighth image, the size comparison is to the grey outlines of four bacilliform bacteria labeled "E. coli". The outlines of two interstellar probes are shown, with circular main housings and protruding instruments and antennae. They are labeled "Voyager 1" and "Voyager 2".]

{{comic discussion}}
[[Category:Math]]
[[Category:Astronomy]]
[[Category:Exoplanets]]