Birthday in an Astronomical Time System

Background:

While civilians find traditional and religious calendars and local times to be useful and comforting, scientists and astronomers seek a calendar and time system that are not subject to local conditions or variations in accounting for months and days.

Astronomers therefore reference their time observations to the time in a pre-defined standard location in Greenwich, England. This global standard time is sometimes referred to as Universal Coordinated Time (UTC).

Astronomers also use a calendar system which measures not months and weeks, but rather the number of days (solar days) that have passed since some pre-defined point in time. The point in time is chosen as Jan 1, 4712 B.C., picked to pre-date a large number of ancient astronomical observations of events such as supernovae and eclipses.

The UTC time is converted to a fraction of a day and combined with the count of solar days since an initial time to create a single decimal number. This way of marking time is called the Julian Date System.

Objective:

To experience astronomical time systems by converting birthdate and time into the Julian Date System.

Description:

The student will convert the local time of his/her birth into UTC. Using their birthdate, they will convert their age into solar days. Using these two values, they will determine their birthdate/time in the Julian Date (astronomical) System.

Items needed:

 

Procedure:

Part I: Write down birth date, birth time, birth location [Example]

  1. Write your birth date in the form YYYY-MM-DD, using numbers, not names, for the month
  2. Write your local birth time in local time format. Round to nearest hour. (HH:00 AM or PM). If unknown, just use local noon.
  3. Write birth location.

Part II: Convert BIRTH TIME to FRACTIONAL DAY in Universal Coordinated Time (UTC) [Example]

  1. Change birth time to an hour between 0 and 24. If born in the morning, just write the hour of birth. If born after noon, add 12 hours to the hour of birth. (8:00 AM = 8 hours, 2:00 PM = 14 hours). If born at midnight, write 0.
  2. Adjust for the time zone of your birth
    • Look up the time zone of your birth location. This can be found here.
    • Find the number at the top or bottom of the time zone chart that corresponds to your birth location.
    • Add this number of hours (if negative, add as a negative number) to your birth time in hours.
  3. Adjust for daylight savings time
    • If born between Apr 4 and Oct 27 in the US, subtract 1 hour
    • Otherwise, add or subtract nothing
  4. You should now have a number of hours somewhere in the range of -13 to 34 hours.
  5. This is the number of hours after midnight (or prior to midnight if your number is negative) in Greenwich, England, that you were born. In other words, this is your BIRTH HOUR IN UTC (Universal Coordinated Time).
  6. As a final step, convert this hour to a FRACTIONAL DAY by dividing by 24. Circle this number.

Part III: Convert BIRTH DATE to AGE IN SOLAR DAYS [Example]

  1. Write today's date in YEAR and DAY-OF-YEAR form:
    • Write the year (i.e. 2007) on your paper.
    • Find the number in the right-hand column of the table below for the month (this is the number of days in the year prior to this month)

    2. Month

    		 Days Prior to Month
    January 0
    February 31
    March 59
    April 90
    May 120
    June 151
    July 181
    August 212
    September 243
    October 273
    November 304
    December 334
    • Add the number of days prior to this month to the day of the month, to get today's day-of-year (i.e. Oct 11 = 273+11=284) and write this number down.
  2. Write your birth date in YEAR and DAY-OF-YEAR form:
    • Write the year of your birth
    • Find the number in the table for days prior to the month of your birth (as we did above.)
    • Add the number of days prior to your birth month to the day of the month of your birth (as we did above.) Write down this number of days.
  3. Find the difference between today's date and your birth date
    • Subtract the year of your birth from the year today (should be a positive number).
    • Subtract the Day-of-Year of your birth from the Day-of-Year of today's date (will be positive if you've had a birthday this year, negative if your birthday is coming up. If it is zero, Happy Birthday!)
    • Multiply the result of subtracting the years by 365.25 (days in a solar year, including leap years)
    • Add to this value the result of subtracting days-of-year (if day-of-year result is negative, add this negative number, keeping the sign)
  4. This would be your age in days if you had been born at midnight in Greenwich, England.
  5. To find your actual AGE IN SOLAR DAYS, add to this number the FRACTIONAL DAY(including the sign) that you computed in Part II, above. Circle this number.

Part IV: Convert birth date and time to Julian Date (Astronomical time system) [Example]

  1. Find today's Julian Date. This can be found at www.onlineconversion.com/julian_date.htm. Enter today's date, and 0 hours, 0 minutes and 0 seconds. (If today is Oct 11, 2007, the Julian Date is 2454384.5).
    1. This is the number of days that have passed since noon, Jan. 1, 4712 B.C. in Greenwich, England.
  2. Subtract your AGE IN SOLAR DAYS from this number
! ! ! This is your ASTRONOMICAL BIRTH DAY ! ! !

Note: This is only approximately correct due to some simplifications and estimations for leap years, daylight savings, etc. For an accurate conversion, visit http://www.onlineconversion.com/julian_date.htm

Example (my values)

Part I - Record birth statistics

 

My Birthdate: 1964-08-18

My Birthtime: 10:00 AM

My Birth Location: Canyon, TX

 

______________________________________________________________________

Part II - Convert birth time to Fractional Day in Universal Coordinated Time

Local time of birth 10 hours
Timezone +6 hours
  16 hours
Daylight Savings Time -1 hour
  15 hours
Convert to Fractional Day 15/24

 

Fractional Day of Birth in UTC = 0.625

______________________________________________________________________

Part III - Convert age to solar days

  Year Month Days prior to month Day of month Day of year
Today 2007 Oct 273 11 284
           
Birthday 1964 Aug 212 18 230
           
Subtract 43       54

(43 years) * (365.25 days/year) + 54 + 0.625 = 15760.375 days

# of Solar Days since my birth = 15760.375

______________________________________________________________________

Part IV - Convert birthdate and birthtime to Julian Date
Days
Julian Date (days)
Note
  Today (midnight)  
2454384.500
Today's Julian Date
Subtract Subtract your age in days
15760.375
2438624.125
! ! ! My Astronomical Birthday is 2438624.125 ! ! !

______________________________________________________________________

____________________________________________________________________