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# Calculation of Julian Date

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### #1 Varah

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Posted 15 July 2020 - 01:44 AM

Hi guys,

This is my first post on the forum. I am not sure if this is the right forum to post it but I guess this might be the right forum. Actually, I am learning to calculate Julian Date manually. I know, there are web sites on the internet that have online calculators but my idea is to learn  and master the procedures instead of being concerned with just getting an answer.

Right now, I am consulting "Practical Astronomy with Your Calculator" by Peter Duffet-Smith (Third Edition) and on page 7 (scanned page attached) he has given the steps to arrive at the Julian date. While his steps are clear but I am basically stuck on the last step (#6) as I see  a value (underlined with red marker) I am not sure how it was derived and there is no explanation.

Thank you.

Varahmihir

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### #2 Arcamigo

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Posted 15 July 2020 - 02:46 AM

1,720,994.5 = 365.25*4716 - 1524.5 where 4716 is a constant that has been added to the formula to avoid trouble for negative years, and 1524.5 is an offset to assure that JD = 0.0 at noon on January 1, -4712 in the Julian Calendar. This is described in Chapter 7 of Jean Meeus' book, Astronomical Algorithms, which you are going to LOVE, if you enjoy problems like this.

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### #3 Varah

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Posted 15 July 2020 - 04:33 AM

Dear Arcamigo,

Thank you very much for this wonderful explanation. I really appreciate your time and trouble for explaining this to me in the most crystal clear manner.

Varahmihir

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Just Be Cuz

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Posted 15 July 2020 - 11:24 AM

Welcome to Cloudy Night Varah!

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### #5 llanitedave

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Posted 15 July 2020 - 03:41 PM

Welcome, Varah!  A most worthy first post!

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### #6 Varah

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Posted 15 July 2020 - 11:21 PM

Dear cuzimthedad and llanitedave, thank you very much for the warm welcome accorded.   I hope the members wouldn't mind, if I occasionally post such questions.

Just Be Cuz

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Posted 16 July 2020 - 01:11 AM

Dear cuzimthedad and llanitedave, thank you very much for the warm welcome accorded.   I hope the members wouldn't mind, if I occasionally post such questions.

Keep asking away Varah. That is what Cloudy Nights is here for!

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### #8 llanitedave

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Posted 16 July 2020 - 05:21 PM

Dear cuzimthedad and llanitedave, thank you very much for the warm welcome accorded.   I hope the members wouldn't mind, if I occasionally post such questions.

"Curiouser and curiouser" is a compliment around here.

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### #9 zjc26138

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Posted 18 July 2020 - 11:19 AM

Really cool first post!

Also welcome to Cloudynights!

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