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Astronomical Time

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Astronomical Time Appendix A Astronomical Time Introduction To do serious astronomy one must have a good understanding of time. Time is simple, right? Actually time is not simple at all, we are just used to it. Take for example there is no zero on clocks. Sure there is a ten which is a 10, but that is ten, not zero. Older clocks use the Roman Numeral X for ten. There is no Roman numeral for zero. Now some may say zero is 00:00:00 h which is midnight. Indeed, some modern time uses a 24 h system and it starts at midnight at 00:00:00 h. Military or Zulu time is like this and in astronomy, Universal Time (UT). All these 24 h times are based on the time at that instant on the Greenwich Meridian (GMT) and start at midnight equal to 00:00:00 h. Some digital clocks allow local time to also be 24 h based and not based on GMT, but local midnight. When I say 00:00:00 h I mean hh:mm:ss , where hh are two digits for the hour (06), mm are two digits for minutes (13) and ss for seconds (33.874564). You can make the time as accurate and precise as you wish by adding numbers to the right of the decimal point in the sec- ond’s part. Time is now 06:13:33.874564 (06 h, 13 min and 33.874564) seconds). Astronomical Times Astronomical Dates Local Standard Time Decimal Date Local Daylight Savings Time Julian Date (JD) Universal Time (UT) Modi fi ed Julian Date (MJD) Sidereal Time Reduced Julian Date (RJD) Local Sidereal Time Heliocentric Julian Date (HJD) J.L. Hopkins, Using Commercial Amateur Astronomical Spectrographs, The Patrick 253 Moore Practical Astronomy Series, DOI 10.1007/ 978-3-319-01442-5, © Springer International Publishing Switzerland 2014 254 Appendix A Astronomical Time and Date Astronomical Time Local Standard Time This is the time in a particular time zone. Time zones are divided up for conve- nience and sometimes very confusing. Slightly North, South, East or West and it could be a different hour. In the continental United States we have 4 time zones Eastern Standard Time (EST), Central Standard Time (CST), Mountain Standard Time (MST) and Paci fi c Standard Time (PST). There are similar time zones around the world. Based on UT, EST is 5 h behind. When UT is 23:00:00 h EST is (−5 h) 18:00:00 h, CST is (−6 h) 17:00:00, MST is (−7 h) 16:00:00 and PST is (−8 h) 15:00:00. Things get confusing when say EST is 20:00:00 as it is 01:00:00 h UT the next day. In astronomy it is always good to supply a double date where the fi rst date is the evening and the second is the next morning’s date. For an observation made on the evening of 29 December 2009 at 18:00:00 h EST the reported date and time would be 29/30 December 2009 23:00:00 UT. Both the UT date and Local date would be 29 December. An observation on the evening of 29 December 2009 at 20:00:00 h EST would be reported as 29/30 December 2009 01:00:00 UT. The UT date would be 30 December, but the Local date 29 December. Local Daylight Savings Time In the continental USA the time zones then become EDT (UT−4), CDT (UT−5), MDT (UT−6) and PDT (UT−7). While this may seem simple, it is not. Not every- where uses Daylight Time. In fact most, but not all, of Arizona never uses Daylight Time so during the months daylight time is in effect, most of Arizona is still on Standard Time (MST) and thus PDT is the same as MST. With proper adjustments all is well. Note: Universal Time does not change with Daylight Savings Time. Universal Time (UT) Also Known a UTC or Coordinated Universal Time In astronomy, most of the time people observe events outside of the Earth. It behooves us to fi nd a standard whereby astronomical events can use the same time. This is where Universal Time or UT comes in. It is the same as Greenwich Mean Time de fi ned as 12:00:00 h when the Sun transits the zero longitude line that passes through Greenwich, England. While this can vary by up to 16 min (due to the equation of time or variation of the Earth’s elliptic speed and axial tilt), UT is now set by an atomic clock (UTC). Generally UTC and UT are the same time. Appendix A 255 Sidereal Time Lines of latitude going north and south of the Equator and lines of Longitude going east and west of the GMT meridian divide the Earth up. From our perspective on Earth, the sky can be thought of as a sphere and divided up similarly. Instead of lines of Latitude and Longitude, the celestial sphere has lines of Declination or Dec (latitude) and Right Ascension or RA (Longitude). The celestial Equator is above the Earth’s Equator. As the Earth rotates on its axis the celestial sphere appears to turn from East to West. The Declination is just like Latitude going from zero degrees at the Equation to +90° at the North Pole and −90° at the South Pole. Longitude or Right Ascension is divided up into 24 h instead of degrees. Because of the Earth’s motion around the Sun a sidereal day is slightly less than 24 h. A sidereal day is 23 h 56 min and 04.091 s. The zero sidereal hour is a merid- ian on the celestial sphere where the Sun crosses the celestial Equator in March. This is also known as the fi rst point of Aries and Vernal Equinox. The RA of a Star is its sidereal time. When that RA time of a star is the same as the local sidereal time, that star is on your meridian. The local time shifts about 2 h a month for the sidereal time. In other words a star is on your meridian at 11:00:00 h UT (04:00:00 h MST) on 15 September. The star will be on the meridian at 09:00:00 h UT (02:00:00) MST) on 15 October and 07:00:00 h UT (00:00:00 h MST) in November. Local Sidereal Time Local Sidereal Time is just the RA that is on your meridian at a given time. Epsilon Aurigae has a RA of 05 h, 02 min. When the star is on your meridian, your Local Sidereal Time is 05:02. Astronomical Date Decimal Date Many applications require the date to be in decimal form. What that means is the day of month followed by a period and then a number representing the percent of the day. For example :12.50000 This would be the 12th day of the month at noon. It does not specify if it is local or UT. That is determined by the application requirement. A more complex time is :19.09444 UT 256 Appendix A This for 18 February 2012 at 19:18 MST which relates to the next day for Universal Time 19 February 2012 and a time of 02:16 which converts to the: 19.09444 UT. In this case the month and year are not speci fi ed. Julian Date uses a similar decimal technique. Julian Date Keeping track of dates over long periods when there are leap years involved and even changes in calendars can be a big problem. This is especially true for astron- omy when you may want to go back in time hundreds of years to an event and know how much time there was between then and now. To make this task easier, a system using Julian Date was devised. Julian Date is the number of days that have elapsed since noon on 1 January 4713 B.C. and is given in the form of decimal days, not in hours, minutes, and seconds. The Julian Day begins at noon Greenwich time or 12 h UT. Leap years and calendar changes do not matter. Examples of Julian Dates are: 11 January 1988 at 2hm 45 UT, JD = 2, 447, 171.6146 11 January 1988 at 12hm 2 UT, JD = 2, 447, 172.0333 What is the origin of the Julian Date system? Contrary to some beliefs, Julian Date has no connection with the Julian calendar and was not named after Julius Caesar. Instead Joseph Justus Scaliger, in 1583, developed the Julian Period. He multiplied the lengths of three cycles: the 28-year solar cycle, the 19-year lunar cycle, and the 15-year cycle of the Roman Indiction (used in calculating the date of Easter). The resulting period (28 × 19 × 15) is 7980 years, which passed through zero in the year 4713 BC. This is a very convenient date because all recorded his- tory, including documented astronomical events, has occurred after this date. Astronomers adopt Julian Dates because the time interval between events is inde- pendent of the day of the week, month, or year. If a table is not available, such as those in the Astronomical Almanac, then Julian Date can be calculated using the formula JD = 367 * Y− Int( 7 * ( Y + Int (( M + 9) / 12 ))) / 4) + Int ( 275 * M / 9 ) + D + 1721013.5 + UT / 24 where Y, M, D, are the year, month, day of month and UT is Universal Time (in 24-h, decimal format).
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