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How Far Is It?
By Bob Bond

One of the perennial star party questions is "How far is it?" where "it" is the thing in the telescope field. This summary information can be used to get a feel for astronomical units and distances.

Astronomical distances vary by many orders of magnitude. The units used to measure the distances are tailored to the distances being measured, just as we use feet to measure our houses and miles to measure our cities here on Earth.

An AU, or Astronomical Unit, is the distance from the Earth to the Sun or about 93 million miles. It is a convenient unit of measure for things in and around the solar system.

A light year is the distance that light travels in one year, or 5.87 * 10^12 miles. It is a convenient unit of measure for nearby stars. Even this unit is small once one begins to talk about the distances to the galaxies.

To relate these two units to each other, consider the following quote from Burnham's Celestial Handbook, volume 1 by Robert Burnham Jr. By a fortunate circumstance, the number of inches in a mile is very nearly equal to the number of AU in one light year, a perfect arrangement for the purpose of constructing mental scale models. Let us imagine a scale model of the solar system, with the Earth represented as a speck one inch away from the Sun. Pluto is then about 3.5 feet away from the Sun. The nearest star on this model will be nearly 4.5 miles away. And all the stars are, on the average, as far from each other as the nearest ones are from us.

The Solar System

Distances are given from the Sun.
 

Body Distance Distance Distance Diameter
Name Million Miles Million KM AU Thousand KM
Mercury 36 58 0.4 4.8
Venus 67 108 0.7 12
Earth 93 150 1 13
Mars 142 228 1.5 6.7
Juno 184 296 2 0.2
Pallas 196 316 2.1 0.6
Ceres 240 387 2.6 1
Jupiter 484 779 5.2 143
Saturn 870 1,400 9.5 120
Uranus 1,802 2,900 19 52
Neptune 2,796 4,500 30 50
Pluto 3,666 5,900 39 2

The Moon is 407,000 KM from the Earth and is 3400 KM in diameter.

Nearby Stars

Distances are given in light years

Note on star names: The bright stars are named by constellation and a Greek letter. The brightest star is usually Alpha, the second brightest Beta, etc. So Vega is also known as Alpha Lyra, the brightest star in the constellation Lyra.
 

Name Mag Distance
Proxima Cen 11.1 4.2
Alp Cen A 0 4.4
Alp Cen B 1.3 4.4
Barnard's star 9.6 6
Wolf 359 13.5 7.8
- 7.5 8.2
LDS 838 12.6 8.6
UV Cet 12.7 8.6
Sirius -1.4 8.6
- 8.4 8.6

Bright Stars

Distances are given in light years
 

Name Star Mag Distance
Rigel Kentaurus Alp Cen A 0 4
Sirius Alp CMa -1.5 9
Procyon Alp CMi 0.4 11
Altair Alp Aql 0.8 16
Vega Alp Lyr 0 25
Arcturus Alp Boo 0 37
Capella Alp Aur 0.1 41
Aldebaran Alp Tau 0.9 67
Antares Alp Sco 1 142
Spica Alp Vir 1 150
Canopus Alp Car -0.7 183
Rigel Bet Ori 0.1 351
Betelgeuse Alp Ori 0.5 333
Deneb Alp Cyg 1.3 1402

Some Messier Objects

Distances are given in light years. 
M# Name  Type  Distance
45 Pleiades  Open Cluster  380
44 Beehive Cluster  Open Cluster  577
7 The Scorpion's Tail  Open Cluster  1,000
27 Dumbbell Nebula  Planetary Nebula  1,250
42 Orion Nebula  Diffuse Nebula  1,600
47 Open Cluster  1,600
20 Trifid Nebula  Diffuse Nebula  2,200
67 Open Cluster  2,250
41 Open Cluster  2,400
97 Owl Nebula  Planetary Nebula  2,600
35 Open Cluster  2,800
21 Open Cluster  3,000
76 Little Dumbbell Nebula  Planetary Nebula  3,400
36 Open Cluster  4,100
57 Ring Nebula  Planetary Nebula  4,100
17 Omega or Swan Nebula  Diffuse Nebula  5,000
1 Crab Nebula  Supernova Remnant  6,300
8 Lagoon Nebula  Diffuse Nebula  6,500
4 Globular Cluster  6,800
16 Eagle Nebula  Open Cluster  7,000
22 Globular Cluster  10,100
71 Globular Cluster  11,700
10 Globular Cluster  13,400
13 Hercules Globular Cluster  Globular Cluster  22,200
5 Globular Cluster  22,800
30 Globular Cluster  24,800
92 Globular Cluster  26,100
80 Globular Cluster  27,400
3 Globular Cluster  30,600
15 Globular Cluster  32,600
54 Globular Cluster  82,800
110 Elliptical Galaxy  2,900,000
32 Elliptical Galaxy  2,900,000
31 Andromeda Galaxy  Spiral Galaxy  2,900,000
33 Triangulum Galaxy  Spiral Galaxy  2,300,000
83 Southern Pinwheel  Spiral Galaxy  10,000,000
81 Bode's Galaxy  Spiral Galaxy  11,000,000
82 Cigar Galaxy  Irregular Galaxy  11,000,000
64 Blackeye Galaxy  Spiral Galaxy  12,000,000
101 Pinwheel Galaxy  Spiral Galaxy  27,000,000
51 Whirlpool Galaxy  Spiral Galaxy  37,000,000
96 Spiral Galaxy  38,000,000
105 Elliptical Galaxy  38,000,000
104 Sombrero Galaxy  Spiral Galaxy  50,000,000
100 Spiral Galaxy  60,000,000
84 Lenticular Galaxy  60,000,000
85 Lenticular Galaxy  60,000,000
86 Lenticular Galaxy  60,000,000

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