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How Many People Share The Exact Same Natal Chart? (And Other Boring Math)

One common argument skeptics have against sun sign horoscopes is that the chances of 1/12th of the population having the same day is extremely low. I have to applaud them for being right, in the same way I applaud a two year old for going potty all by themselves. That there can be commonalities between people based on their birth month is something science actually has no problem with. But the reality is that most astrologers don’t consider astrology to be as simplistic as its critics charge. Just as every person is a unique constellation of genetics, environment and individual nature, every chart is individualized by various factors: sign placement of the planets, house placement of the planets, aspects between planets, and of course the degree of the Ascendant/Descendant and MC/IC axes and the Lots.

The Ascendant is probably the primary way charts are individualized. Plenty of people around the world are born within the same minute as each other, and while they would have the same aspects and planetary sign positions, the Ascendant and house positions would be totally different. The Earth turns one degree of longitude every 4 minutes, thus the Ascendant shifts one degree every 4 minutes. Even a difference of 4 minutes between birth times can result in significant differences for when transits perfect, especially with slower-moving planets. To really have the same chart, you’d have to be born within the same minute and close to the same location. This got me thinking, how many people actually share the same chart? What are the chances someone was born at the same time and place as you?

Get ready for some terrible math.

One way to answer this question is to consider the number of minutes in a calendrical non-leap year, which is 525,600. I’ve been waiting to casually drop that bit of trivia my whole life. Finally, watching Rent paid off. I say a calendrical non-leap year in order to pre-empt the dick in the comments who will remind us all that there are 525,948.7666666666666667 minutes in a tropical year. I’ve already specified what kind of year, so can it, dick in the comments. And somehow I just think that number wouldn’t make as catchy a lyric in Seasons of Love.

Just in case you’re not getting my stupid jokes about this film.

So anyway. You divide 525,600 by the number of births in a given year and location, and voila, you have an evenly distributed rate of births per minute, and the rough likelihood of two people sharing the same sign and degree Ascendant. This has some obvious but thought-provoking implications. As a location’s annual number of births approaches 525,600, the closer you would get to the rate of 1 birth a minute. As the global population continues to climb, this may become a more common phenomenon.

But what do I mean by a location? Since 1 degree of longitude roughly corresponds to 69 miles, for two births to have the best chance at being born at the same time with the same degree Ascendant, they’d have to be within 4 minutes and within 69 miles east or west of each other. 1 degree of latitude roughly corresponds to 69 miles, but it only affects the Ascendant by half a degree, so that means the two births would have to take place within 138 miles north or south of each other. This amounts to an area of 9522.06 square miles, which is 0.38% of all habitable land on Earth, barring some sort of Waterworld scenario in which case that percentage would be even smaller. Please God don’t ever let there be a Waterworld scenario.

So to recap, in an area of 9522.06 square miles, where the number of births per year is at least 131,400 (one birth every 4 minutes), that is where you’d find the greatest likelihood of two births occurring on the same day with the same degree Ascendant. Here’s what that area looks like, using my own birth location as an example:

same degree area
Maybe we could call this a “Rising Rectangle”?

But what about births in general? Most of my readers are Americans, so I’m catering to you this time. Sorry, rest of the world. The contiguous United States of Murica is about 3,800,000 square miles, divided by 9522.06 comes out to 399 distinct patches or “Rising Rectangles” where births occurring within 4 minutes will have Ascendants within 1 degree of each other. In 2013, there were 3,932,181 births. There were 62.5 births per 1000 persons, and there are 84 persons per square mile on average. This means there are 1000 people every 12 square miles, and consequently 49,594 births per “Rising Rectangle” in a year. Divide by Rent and you get 10-11 minutes between each birth within a “Rising Rectangle”.

If I have reasoned correctly, and there may be every reason to doubt that, Ascendants of people born on the same day close to each other in the USA are between 2-3 degrees apart. In areas where the number of births overall is higher, it’ll be less than that, and vice versa. You will just have to look up the birth statistics of a particular place in order to get a meaningful answer to this question.

In any case, there are at least 2 people who were likely born very close to your birth time, the one right before you and the one right after you. DERP. Next time you hear a “1/12th of the population” complaint, agree with them and tell them it’s more like ≤3/7,125,000,000th. And then congratulate them for going potty all by themselves.

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7 Replies to “How Many People Share The Exact Same Natal Chart? (And Other Boring Math)

  1. Thanks so much Patrick! I appreciate all of your calculations. I always like having a good response at the ready for skeptics. 🙂
    I do wish there was some sort of registry for people into astrology for those born on the same day same year with same rising and moon. Of course exact time matters, but I would still find it interesting. Thanks!!

  2. It is Friday, March 2nd 2018. Before i met my current girlfriend i was talking to someone on and off for about 3 years.. we shared the exact same houses in our birthchart.. Today me and my gf were scrolling through her snapchat memories, she had a picture of her sisters chart.. not only do me and her sister share the exact same houses, but we share all the same planets (except for her moon sign and venus were in different signs) BUT WHAT ARE THE ODDS??? i want to know what my connection to them is, and why i’ve met two people who are extremely close to me that have the same chart aspects..

    1. What transits are you all having? If there are similar transits you’re having between you then they are likely agents for those transits.

  3. My twin flame and I were born 8 years, 88 days apart, and have all the same exact planets in our houses.

    What the? How rare is this?

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