# Gravitational influence of other planets?

I have a friend who believes in astrology and she insists that the gravitational influence of the planets has an effect on the child when it's born. The last time she was going on about this a thought occurred to me. I wonder which is stronger, the gravitational effect of heavenly objects or the the gravitational effects of the delivery room staff? Any thoughts on this would be appreciated. Thanks.

• This is an old thought, probably going back to the 1970s and Carl Sagan. Answer "it depends" the-orbit.net/lousycanuck/2011/09/17/… Aug 11 '21 at 11:34
• It may be worth noting that people who believe in astrology are unlikely to be swayed by an appeal to reason. There isn't much point in proving your point. The most you are likely to achieve is the loss of a friend. Aug 11 '21 at 15:48

Actually the gravitational tug from a nearby planet is stronger than that from a nearby person. Venus is the planet that comes closest to us - it's 42 billion meters away when it's between us and the sun. Its mass is $$4.9 \times 10^{24}$$ kg.

Someone close in the delivery room might be 1 meter away and weigh 80 kg. The ratio of Venus' tug to an obstetrician's is

$$\frac{4.9 \times 10^{24}}{(42 \times 10^{9})^2} \ \diagup \ \frac{80}{1^2}$$

or roughly 35 times as strong. Even when Venus is most distant its pull is about equal to that of a nearby person.

• There's an error in your calculation; at closest approach it's 35, not 2000. At farthest it's ~1, not 53. If the obstetrician actually needs to touch the child, a reasonable assumption I suppose, the results will be even lower…
– pela
Aug 11 '21 at 18:44
• This answer is incorrect because the Earth as a whole is also attracted to Venus. It's the difference between the attraction of the Earth as a whole toward Venus versus the attraction of a point on the surface of the Earth (e.g. a newborn baby) toward Venus that matters. This is very, very, very small. Aug 12 '21 at 16:59
• -1 for using a broken calculator. Aug 12 '21 at 20:52
• @uhoh I can use Mathjax and I have used it on other Stack Exchange sites, but I couldn't find a way to use it here. I could have found a way, but I didn't think it was worth the effort. No one answered the question for several hours after it was posed, and I thought, still think, it's an easy high school physics problem. Aug 13 '21 at 2:20
• You and @pela: I never should have changed from using a slide rule to using calculators and spreadsheets. Corrected the arithmetic. Aug 13 '21 at 13:23