# About julian days

1. Jun 15, 2007

### lewis198

the julian day formula goes like this:

$$\begin{matrix}a & = & \left\lfloor\frac{14 - month}{12}\right\rfloor \\ \\y & = & year + 4800 - a \\ \\m & = & month + 12a - 3 \\\end{matrix}$$

For a date in the Gregorian calendar (at noon):

$$\begin{matrix}JDN & = & day + \left\lfloor\frac{153m + 2}{5}\right\rfloor + 365y + \left\lfloor\frac{y}{4}\right\rfloor - \left\lfloor\frac{y}{100}\right\rfloor + \left\lfloor\frac{y}{400}\right\rfloor - 32045\end{matrix}$$

For a date in the Julian calendar (at noon):

$$\begin{matrix}JDN & = & day + \left\lfloor\frac{153m + 2}{5}\right\rfloor + 365y + \left\lfloor\frac{y}{4}\right\rfloor - 32083\end{matrix}$$

I do not understand how each step works, so can someone please guide me through?

Last edited by a moderator: Jun 22, 2007
2. Jun 15, 2007

### D H

Staff Emeritus
It looks like the latter two are formulae for some kind of modified Julian date rather than a Julian date. They also don't like right. Where did you get these? Much better formulae are at the http://aa.usno.navy.mil/faq/docs/JD_Formula.html" [Broken].

Last edited by a moderator: May 2, 2017
3. Jun 21, 2007

### heafnerj

See

http://www.vsg.cape.com/~pbaum/date/date0.htm [Broken]

for a derivation.

Last edited by a moderator: May 2, 2017
4. Jun 22, 2007

### Chronos

Another source to consider:
http://aa.usno.navy.mil/faq/docs/JD_Formula.html [Broken]

Last edited by a moderator: May 2, 2017
5. Jun 24, 2007

thanks guys.