unitsコマンドで3ミリ光秒の到達距離を調べたりA4用紙何枚で一畳になるか調べたりする

unitsコマンドは単位から単位の簡易計算に使えるツールです
A4用紙のサイズといった規格から、mからマイルへという変換も簡単にできます

実際のところ

例として3ミリ光秒*1とマイルの対応なんていう
(すくなくとも私は)突然新橋の路上で聞かれても回答できそうもない話も

# units "3 millilightseconds" miles
        * 558.84719
        / 0.0017893979

よく使う規格も定義されてますのでA4のサイズが見たいときには

# units "A4paper"
        Definition: 210 mm 297 mm = 0.06237 m^2

さらに中々凄まじい事に、日本ローカルな坪や尺貫法、古代ローマの単位なんてのまで定義されています。

# units "4.5 edoma" m2
        * 6.9504132
        / 0.14387634

# units "4.5 jou_area" m2
        * 6.9504132
        / 0.14387634

約25枚のA4紙は約1畳(江戸間)！

# units "1 edoma" A4paper
        * 24.76409
        / 0.040381052

何故か指輪のサイズまでわかる

日本規格の指輪の内径サイズまで分かります……ナンデ？？？

# units "jpringsize(10)" cm
        * 5.0265482
        / 0.19894368

定義をみると……結構複雑なことをやっている

# Japanese sizes start with size 1 at a 13mm inside diameter and each size is
# 1|3 mm larger in diameter than the previous one.  They are multiplied by pi
# to give circumference.

jpringsize(n)  units=[1;mm] domain=[1,) range=[0.040840704,) \
               (38|3 + n/3) pi mm ; 3 jpringsize/ pi mm + (-38)

蛇足

あまりに面白すぎたので日本部分の定義ファイルを転載

#
# Traditional Japanese units (shakkanhou)
#
# The traditional system of weights and measures is called shakkanhou from the
# shaku and the ken.  Japan accepted SI units in 1891 and legalized conversions
# to the traditional system.  In 1909 the inch-pound system was also legalized,
# so Japan had three legally approved systems.  A change to the metric system
# started in 1921 but there was a lot of resistance.  The Measurement Law of
# October 1999 prohibits sales in anything but SI units.  However, the old
# units still live on in construction and as the basis for paper sizes of books
# and tools used for handicrafts.
#
# Note that units below use the Hepburn romanization system.  Some other
# systems would render "mou", "jou", and "chou" as "mo", "jo" and "cho".
#
#
# http://hiramatu-hifuka.com/onyak/onyindx.html

# Japanese Proportions.  These are still in everyday use.  They also
# get used as units to represent the proportion of the standard unit.

wari_proportion      1|10
wari                 wari_proportion
bu_proportion        1|100    # The character bu can also be read fun or bun
                              # but usually "bu" is used for units.
rin_proportion       1|1000
mou_proportion       1|10000


# Japanese Length Measures
#
# The length system is called kanejaku or
# square and originated in China.  It was
# adopted as Japan's official measure in 701
# by the Taiho Code.  This system is still in
# common use in architecture and clothing.

shaku              1|3.3 m
mou                1|10000 shaku
rin                1|1000 shaku
bu_distance        1|100 shaku
sun                1|10 shaku
jou_distance       10 shaku
jou                jou_distance

kanejakusun        sun      # Alias to emphasize architectural name
kanejaku           shaku
kanejakujou        jou

# http://en.wikipedia.org/wiki/Taiwanese_units_of_measurement
taichi             shaku   # http://zh.wikipedia.org/wiki/台尺
taicun             sun     # http://zh.wikipedia.org/wiki/台制
!utf8
台尺               taichi  # via Hanyu Pinyin romanizations
台寸               taicun
!endutf8

# In context of clothing, shaku is different from architecture
# http://www.scinet.co.jp/sci/sanwa/kakizaki-essay54.html

kujirajaku         10|8 shaku
kujirajakusun      1|10 kujirajaku
kujirajakubu       1|100 kujirajaku
kujirajakujou      10 kujirajaku
tan_distance       3 kujirajakujou

ken                6 shaku  # Also sometimes 6.3, 6.5, or 6.6
                            # http://www.homarewood.co.jp/syakusun.htm

# mostly unused
chou_distance      60 ken
chou               chou_distance
ri                 36 chou

# Japanese Area Measures

# Tsubo is still used for land size, though the others are more
# recognized by their homonyms in the other measurements.

gou_area             1|10 tsubo
tsubo                36 shaku^2    # Size of two tatami = ken^2 ??
se                   30 tsubo
tan_area             10 se
chou_area            10 tan_area

# http://en.wikipedia.org/wiki/Taiwanese_units_of_measurement
ping                 tsubo     # http://zh.wikipedia.org/wiki/坪
jia                  2934 ping # http://zh.wikipedia.org/wiki/甲_(单位)
fen                  1|10 jia  # http://zh.wikipedia.org/wiki/分
fen_area             1|10 jia  # Protection against future collisions
!utf8
坪                   ping      # via Hanyu Pinyin romanizations
甲                   jia
分                   fen
分地                 fen_area  # Protection against future collisions
!endutf8

# Japanese architecture is based on a "standard" size of tatami mat.
# Room sizes today are given in number of tatami, and this number
# determines the spacing between colums and hence sizes of sliding
# doors and paper screens.  However, every region has its own slightly
# different tatami size.  Edoma, used in and around Tokyo and
# Hokkaido, is becoming a nationwide standard.  Kyouma is used around
# Kyoto, Osaka and Kyuushu, and Chuukyouma is used around Nagoya.
# Note that the tatami all have the aspect ratio 2:1 so that the mats
# can tile the room with some of them turned 90 degrees.
#
# http://www.moon2.net/tatami/infotatami/structure.html

edoma                (5.8*2.9) shaku^2
kyouma               (6.3*3.15) shaku^2
chuukyouma           (6*3) shaku^2
jou_area             edoma
tatami               jou_area

# Japanese Volume Measures

# The "shou" is still used for such things as alcohol and seasonings.
# Large quantities of paint are still purchased in terms of "to".

shaku_volume         1|10 gou_volume
gou_volume           1|10 shou
gou                  gou_volume
shou                 (4.9*4.9*2.7) sun^3   # The character shou which is
                                           # the same as masu refers to a
                                           # rectangular wooden cup used to
                                           # measure liquids and cereal.
                                           # Sake is sometimes served in a masu
                                           # Note that it happens to be
                                           # EXACTLY 7^4/11^3 liters.
to                   10 shou
koku                 10 to  # No longer used; historically a measure of rice

# Japanese Weight Measures
#
# http://wyoming.hp.infoseek.co.jp/zatugaku/zamoney.html

# Not really used anymore.

rin_weight           1|10 bu_weight
bu_weight            1|10 monme
fun                  1|10 monme
monme                momme
kin                  160 monme
kan                  1000 monme
kwan                 kan         # This was the old pronounciation of the unit.
                                 # The old spelling persisted a few centuries
                                 # longer and was not changed until around
                                 # 1950.