Barycentric Julian Date (Inputs)

This page describes the inputs required to run our applet, which converts from UTC time to BJD in TDB with a precision of a few ms. A description of what this calculation entails can be found here.

UTCs - A list of up to 10,000 UTCs to convert to BJD_TDBs. These will typically be the date/times at the middle of your exposure. They can be in JD format (e.g., 2455335.28010416), YYYY MM DD HH MM SS.SS (e.g., 2010 05 18 18 43 21.432), or any combination of the two. All times must be between Dec 15, 1949 (2433266.0) and Dec 31, 2049 (2469807.0). If using a space observatory, you must make sure the spacecraft was in flight during the times you specify, or the telnet session will hang. Generally, HORIZONS cannot extrapolate spacecraft positions into the future. If 10,000 dates is a serious limitation to you, send me an email and let me know. It is not a fundamental problem, but will require some effort on my part to bypass, and would be worth it if there is enough interest.

RA/Dec - The J2000 Right Ascension and Declination. RA must be between 0 and 24 hours (0 and 360 degrees), and the dec must be between -90 and 90 degrees. There are several allowed formats -- all the following are equivalent ways to input the coordinates of HAT-P-4b:



Dec (J2000)
15 19 58.000
229 14 52.506
36 13 47.0
15 19.96667
36 13.78333
229 14.8751

Note: An error of 0.25" in the object's coordinates will result in an error as large as 1 ms.

-- Optional Inputs --

If none of these are specified, we will assume the observer is at the geocenter, and the results will be accurate to 21.3 ms. If more than one are selected, "Space Observatory" has highest precedence, then "Earth-Based Observatory", then Latitude/Longitude/Elevation.

Earth-Based Observatory - A drop down menu of observatories. This will automatically retrieve the Latitude, Longitude, and Elevation of the observatory as defined in from the IDL astronomy library, last updated in July 2008 (using a list of observatories from noao$lib/obsdb.dat file in IRAF 2.11).

NOTE: contains the **West** longitudes, which we convert to East longitude before we use it. Do not enter the longitude from directly in the longitude box in this applet.

Latitude - The latitude of observatory, in decimal degrees. Must be between -90 and 90 degrees. Latitude and Longitude must be specified or both will be ignored.

Longitude - The East longitude of observatory, in decimal degrees (E Longitude = 360 - W Longitude). Must be between -360 and 360 degrees. Latitude and Longitude must be specified or both will be ignored.

Elevation - The Elevation of the observatory, in meters. Must be between 0 and 10000 meters. This can be left blank with a maximum error of 30 us (less than the accuracy of this applet). It is left here for consistency with "Earth-based Observatory", which uses the elevation from

Space Observatory - A drop down of Space observatories, organized by NAIF ID codes, which are organized by date launched. We moved the most popular space observatories (Hubble, Spitzer, Kepler, EPOXI, Chandra, Hershel, and Plank) to the top, however. If selected, the PHP script will begin a telnet session to HORIZONS to retrieve the spacecraft's ephemeris in 10 minute intervals from (minimum date - 30 minutes) to (maximum date + 30 minutes). This ephemeris will be interpolated to the desired times by our IDL program to generate the BJD_TDBs. The telnet session cannot return more than 90,000 dates/positions. If the date range is sufficiently large (~1.7 years) that a 10 minute interval will result a query that is too large, we will scale the interval to match, but issue a warning about a possible loss in precision. A 10 minute step size is sufficient for 1 ms precision of an Earth-orbiting satellite. Depending on how quickly the satellite moves, a larger step size may result in a loss of precision. This can be avoided by reducing the range of input dates to less than 1.7 years. Be warned that the telnet session takes between 1 and 2 minutes and is somewhat flaky. However, if it completes, the results are trustworthy.

Verbose - If checked, and a Space Observatory is selected, we will print the telnet session in progress. Note that some browsers (Chrome) buffer the output and will not display in real time. Firefox and IE8 have been tested and work well. This is useful if you want to save the ephemeris we generate from HORIZONS, or for debugging if the script does not complete as expected in 2 minutes (if the last thing displayed was "Starting CT" or "Ending CT", your times were probably out of range).

User inputs are NOT logged in order to ensure your privacy and respect the potential proprietary nature of your observations. We record the time of query and your browser for usage statistics only. If you notice any unexpected behavior, please contact me with your inputs and expected output.

Copyright © Jason Eastman () All Rights Reserved.
Last Updated: May 18, 2010