Understanding Timings used in the Journal

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    Robert Wiltshire

    I’ve been a member for too long without asking this question.

    What is this digital time format that is used by the BAA?

    In the recent journal on page 337 there is a table on Lunar Occultations. The time in UT seems fairly normal except that on a closer look two of the times have unusual digital points to them. On Nov 9 at 10:47.7 and Nov 21 at 19:48.9. What is this 0.7 and 0.9 time?
    It can’t be seconds as these are counted up to 60. So 10:47.7 could be at 10am, 47 minutes and 42 seconds. If the 0.7 is 7/10th of a minute. So why not say 10:47:42?
    What is this time called because I’ve been looking everywhere here and elsewhere for a description of this time. It’s really difficult to try and describe it for create a meaningful search. What clocks or devices use this unusual time so that it can be read?

    Thanks for any help clearing it up.

    Dr Paul Leyland

    I can’t answer your question authoritatively but will point out that the precision of 10:47.7 and 10:47:42 differ six-fold.

    If a calculated value is known only to 0.1 minutes it is often not a good idea to specify it to a precision of 1 second as that would give a false impression as to how well determined is the true value.


    I, also, find these timings very confusing. And often wonder. Is it absolutely necessary?

    David Arditti

    It is decimals of a minute. So e.g. 10:47.7 is 10:47 and 7/10 of a minute, which is 42 seconds.

    Some software packages used in astronomy produce this output.

    I observe Jupiter a lot and use the WinJupos package by Grischa Hahn, and that gives everything in decimals of a minute. So also every piece of software that feeds into this (e.g. the image capture software) has to use that time format as well. In this case I think the reason is just historical, that visual observations could not be accurate to better than this, and in early days of programming it was easier to do it this way, and it has carried on. I don’t really like it.

    Occultations of course can be timed to at least a tenth of a second, but I suppose the prediction does not need to be so accurate, so I suspect this is really due to both trying to save space (in print) and software that is a bit long in the tooth.

    But someone who works with occultation predictions might care to comment.

    Tim Haymes

    The occultation times in the table on page 337 are for an observer at Greenwich. See the HBAA page 39 and note under the table. Why the 0.1 minute precision? Well this is the format (as suggested by David) that is produced by Occult4 software. There is a routine that produced the tabulated data used by the Handbook. This format is historical, and goes back to the 1960s, but a good summary for two stations.

    Predicted times can be computed to better than 1 sec if the observer coordinates are used in the software, so the tabulated times are only a guide. For other locations the times will differ by up to 2 minutes depending on how far the observer is from Greenwich.

    Im grateful to Nick Hewitt for including Lunar prediction on his Sky Notes page. I find them fascinating to observe. The Lunar Section Circular contains predictions to 8th magnitude for those seeking more opportunities. The time format for these are dd hh mm ss.s for a single station – again a guide only.

    I offer a service to keen occultation observers. In that i can produce predictions to 1 sec precision for their site via email.

    Tim –
    Lunar Section.

    • This reply was modified 7 months, 4 weeks ago by Tim Haymes.
    Robert Wiltshire

    So pleased to hear it is not just me then.

    Robert Wiltshire

    So the decimal seconds is as I thought. And for a tabulated set of values to be used in an observation I can see it would make sense. With the software, at least.

    So now there is the use of another decimal format in written text. Decimal minutes. In the same journal page 282. In the paragraph ‘The Outburst’ on the first line. How do you read 2023 Jul 20.82? In your head can anyone understand at what time that is? Without resorting to a calculator I guess it’s somewhere after 10 minutes to nine. This isn’t tabulated information for accuracy this is conversational text. Or am I just being picky?

    Robert Wiltshire

    Thank you for the explanation. And glad that those other timings you mention are in a more conventional H:M:S.s , a more useful and conventional format.

    Gary Poyner

    In VS observing (and Comets too), it’s normal to give the decimal of the day following the days date. So Jul 20.82 is July 20, 19h 45m UT.

    The time is written down in hours and minutes at the telescope, then the spreadsheet works out the decimal of the day when the observations are typed up. To obtain the decimal of the day, my spreadsheet multiplies the hour by 60, adds the minutes, then divides by 1440. This has been normal practice in VS observing for many years – particularly CV’s (although some observers still do report in hours and minutes).

    I tend to report everything in decimal of the day these days.


    Nick James

    Robert. You’ll find quite a lot of different date and time formats are used in astronomy. The decimal date format is quite common for things that vary (such as outbursting comets) since it allows fairly quick mental calculation of the time between events. The next step is to get rid of years, months and days altogether and quote in terms of Julian Date. This is used a lot too but it is more difficult for the average human to interpret. If we arranged to meet at a pub at JD 2460254.333 most people (except some variable star observers) would have to go and look up a more familiar date before leaving home.

    The point made by Paul Leyland up the thread is important too. In science, when we talk about quantities, we don’t want to imply more precision than is actually present. If you took it literally, July 20.82 would be 19:40:48 but we clearly don’t know the time of the outburst to that precision. The use of two decimal places in this context is used to indicate the level of precision that we think we have, i.e. around one hundredth of a day or around 15 minutes.

    Dr Paul Leyland

    I tend to report everything in decimal of the day these days.

    I report everything in JD but convert to year/month/decimal day when sending material to The Astronomer because that’s the format preferred there.

    The conversion function is really rather cute but I won’t quote it here, unless requested, because the complexity imposed by irregularly long months and years makes for a very baroque looking function.

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