# Lightbulb Brightness (re: Distance of Celestial Bodies)

**URL:** <https://boards.straightdope.com/t/lightbulb-brightness-re-distance-of-celestial-bodies/41807>\
**Category:** Cecil's Columns/Staff Reports\
**Created:** [November 19, 2000, 2:54am UTC](https://boards.straightdope.com/t/lightbulb-brightness-re-distance-of-celestial-bodies/41807 "2000-11-19T02:54:49Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![cjclark](https://avatars.discourse-cdn.com/v4/letter/c/7bcc69/32.png) [@cjclark](https://boards.straightdope.com/u/cjclark)\
**Post date:** [November 19, 2000, 2:54am UTC](https://boards.straightdope.com/t/lightbulb-brightness-re-distance-of-celestial-bodies/41807/1 "2000-11-19T02:54:49Z")

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On 07-Nov-2000, Chronos wrote in his answer to, “How do  
they figure the distance between celestial bodies?”

The apparent brightness of an object depends both on  
how bright it actually is, and on how far away it is.  
For example, if a hundred watt light bulb is 10 meters  
away, it’ll look exactly as bright as a 25-watt light  
bulb 5 meters away.

I just wanted to point out that the assumption that a  
100-watt light bulb is not four-times “as bright” as  
as a 25-watt bulb. That is, lumens per watt is not  
constant as wattage changes. This is especially true  
when considering different types of bulbs, e.g.  
incandescent versus fluorescent, but is also true for  
a 25-watt incandescent versus 100-watt incandescent.

Go to your favorite web search engine and do a search  
on “watts” and “lumens” for any number of references  
on the topic. Or go to the utility closest and look at  
the wattages and luminosity numbers printed on the front.

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**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\
**Post date:** [November 19, 2000, 6:47am UTC](https://boards.straightdope.com/t/lightbulb-brightness-re-distance-of-celestial-bodies/41807/2 "2000-11-19T06:47:45Z")

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The original column: [How do they figure the distances between celestial bodies?](http://www.straightdope.com/mailbag/mstardistance.html)  
It’s a good idea to include the link, so that we can make sure that everyone’s on the same page, and so **CKDextHavn** doesn’t yell at you.

As for your comment, how about if I weasel out and say I was refering to bolometric luminosity, which is totalled over the entire electromagnetic spectrum? Neglecting the amount of heat that’s lost via conduction and convection, that’ll be exactly equal to the wattage of the bulb. You do have a good point, though, I should have been a little more careful with that when writing it (and would have been, had I realized that there was such variation).