Unveiling the Modified Julian Calendar & Galactic Coordinates

1. Objectives • Understand what the Modified Julian Calendar is and why it was made. • Be able to tell today’s date in the Modified Julian Calendar. • Understand Universal Transverse Mercator. • Be familiar with Celestial and Galactic Coordinate Systems. • Be able to find and place objects on a Galactic Coordinate map • Understand why these are used in Cosmic Ray detection.

2. Modified Julian Calendar • In 1583 a French scholar, Joseph Justus Scaliger, invented the Julian Period. • It was designed to replace Western calendars which were based on the ruling eras of nation.

3. Modified Julian Calendar • The Julian period is equal to 7980 years of exactly 365.25 days • 7980 is based off of the time it takes for three common cycles of the time to align again at (1,1,1) • 28 year solar cycle • 19 year lunar cycle • 15 year Roman indiction cycle 28 (Solar) * 19 (Lunar) * 15 (Indiction) = 7980

4. Modified Julian Calendar • The solar cycle is the time that it takes for calendar days of years to repeat. • Thus you only ever need to own 28 consecutive calendars.

5. Modified Julian Calendar • The lunar cycle is the time it takes for the moon to be approximately the same phase at the beginning of the year • In other words there are 19 years in between years that begin with a full moon

6. Modified Julian Calendar • The indiction cycle is an old Roman tax period • It has historical importance because dates were often recorded using this cycle • No one knows where it came from

7. Modified Julian Calendar • Scaliger determined what year the solar and indiction cycles were in in the year of Christ’s birth • The solar cycle was in the 9th year • The indiction cycle was in the 3rd year • The lunar cycle was set to 1 on that year and so year 1 in our calendar became (9,1,3) or the 4713rd year • The year 4713 B.C. then became the first year in the Julian Period

8. Modified Julian Calendar • The calendar started at 0 at noon on January 1st 4713 B.C. • Each day adds 1 to the count with partial days adding a partial count • i.e. 12 hours adds .5 to the count • 3 hours adds .125 • 18 hours and 30 min. adds .77083 • Today’s date is very long • 2451809.5 is midnight Sept. 22, 2000 • Recently the Modified Julian Calendar has been put into use

9. Modified Julian Calendar • The modified Julian calendar has 2 primary benefits • The Julian day number is reduced in length by at least 2 digits • The day begins at midnight instead of noon • The modified Julian calendar day equals the Julian calendar day minus 2400000.5 • Zero is now midnight Nov. 17, 1858 • All dates there after are consecutive just like the Julian Calendar

10. Why Do We Care?This Won’t Ever Come Up! (Will It?) • On June 15, 763 B.C. there was a solar eclipse seen at Nineveh. • On April 14 425 B.C. there was a lunar eclipse seen at Nineveh. • How many days elapsed between these two celestial events?

11. Same Example Using Julian Days • On the Julian Date of 1,442,903 there was a solar eclipse seen at Nineveh. • On the Julian Date of 1,566,296 there was a solar eclipse seen at Nineveh. • Now it is easy to see that there were 123,393 days between these two recorded celestial events.

12. The Galaxy Awaits • The Earth is a wonderful place and all, but how can we locate things that aren’t on Earth. • One of the major issues currently with Cosmic Rays is determining where they come from. • Once we determine where they come from, how do we keep track of that location? A very perplexing problem!

13. Don’t Worry Homer • Luckily for us, Astronomers have devised ways to locate and map objects. • It is called Galactic Coordinates. And it looks like this:

14. Our Galaxy(Aren’t We Pretty?) Galaxy Center

15. Latitude Measurement

16. Longitude Measurement

17. The Poles Defined(Well, Shown Anyway)