Why use the Hipparcos/Tycho data?
Guide was the first commercially-available software to use the Hipparcos/Tycho data as the basis for data about brighter stars. The question often arose (and occasionally still arises) as to what benefits this dataset really provides over previous star catalogs such as PPM, SAO, and the GSC.
The short answer is that, for many people, the improvements in Hipparcos/Tycho are not particularly useful. For others, they are extremely useful. Following are some of the major differences.
Hipparcos/Tycho provides accurate parallax (and therefore, distance and physical) data
One of the major reasons the European Space Agency went to the considerable effort to conduct the Hipparcos mission was to get good data on stellar distances. Prior to this catalog, data on distances was rare and of poor quality. The PPM, SAO, and GSC catalogs don't provide any parallax data at all; and even if they did, the quality would be doomed to be quite low.
The Hipparcos data gives decent distance data out to several hundred parsecs (the quality degrades with distance; the distance to Barnard's Star is determined to a fraction of a percent, but the distance to Deneb is essentially a guesstimate.) And accurate distances mean accurate luminosity and magnitude data.
For example, following is the distance-related section of the data Guide gives for Betelgeuse, based on Hipparcos data:
Distance: 131 +/- 40 parsecs (430 +/- 130 light-years) Luminosity: 9400 +/- 5700 times that of the sun Absolute magnitude: -5.14 +/- 0.65
Most of the advantages of Hipparcos/Tycho are in the category of "more accurate". But this data is in the category of "completely new".
Hipparcos/Tycho provides accurate data for far more stars than other catalogs
Hipparcos/Tycho gives good positional and magnitude data for over 1 million stars, over twice as many as the best runner-up (PPM). There are some catalogs providing data on more stars, such as the GSC, Astrographic Catalog (AC 2000), and the USNO A1.0 Catalog. But none of these provide nearly the precision, in position and especially for magnitude data, that the Hipparcos/Tycho catalogs do... though to be fair, no catalog provides the level of precision that Hipparcos/Tycho does.
Hipparcos/Tycho provides more consistent data
One pretty serious problem with all previous catalogs is that they often omitted bright stars, while still collecting data about dimmer stars. This was not entirely accidental. The purpose of the catalogs such as the SAO, PPM, and GSC was to get a certain number of stars per square degree. Once they got that number, they stopped looking. (This is why GSC-based programs, including Guide, go to magnitude 15 or so in sparse regions like Virgo, but don't go down much past 13.5 in dense regions such as Sagittarius.)
So these catalogs are good for providing an astrometric reference, but they aren't quite so good for making star charts. On the other hand, the Tycho catalog was intended to just do a survey, a "catalog 'em all, let God sort 'em out" approach that is mostly complete to about visual (V) magnitude 10.5 (though it does contain fainter stars down to about magnitude 11.5).
Hipparcos/Tycho provides more precise magnitude data
Earlier catalogs such as SAO, PPM, and GSC generally provided quite imprecise magnitude data. That followed partly from the fact that none were even intended to provide good magnitude data, and partly from the fact that doing good photometry is a difficult task at best, especially when done from underneath an atmosphere.
Furthermore, the magnitude data in these catalogs was derived from photographic plates, which don't react in quite the same manner as the human eye. The result is that the magnitude data in these catalogs is not very useful for magnitude estimates of asteroids, comets, and variable stars. (In particular, one is well-advised to just forget about using GSC magnitudes for anything whatsoever.)
Hipparcos/Tycho, on the other hand, was intended (in part) to measure magnitudes accurately enough to give us a better understanding of variable stars. It therefore gives VT ("visual on the Tycho system") and BT ("blue on the Tycho system") magnitudes for each star, and Hp ("Hipparcos photometric") magnitudes for those stars in the Hipparcos section of the catalog. And almost without exception, these magnitudes are given to much higher accuracy than anything previously available.
Hipparcos/Tycho provides more precise positional (astrometric) data
As mentioned above, the SAO, PPM, and GSC catalogs were all intended to have good positional accuracy. But there are limits as to just how good positional data gathered from scanned plates can be. GSC has errors of the rough order of a full arcsecond in places. PPM is far better, with RMS errors of about .2 arcsecond.
In almost all cases, Hipparcos/Tycho data is far more precise than this. For many people, this is immaterial. Dots printed on paper star charts will be corrected by a few thousandths of an inch, and no merely human observer would see the difference. People doing astrometry with CCD cameras should be very interested... though, sad to say, the field of an average CCD camera is not always going to contain enough Tycho/Hipparcos stars to make it possible to do astrometry.
Also, people working on asteroid occultations of stars should be very interested. Previously, these predictions were not all that reliable. Use of Tycho data improves matters (and use of Tycho data with ACT proper motions improves matters even further), so that we are limited by errors in the asteroid position now instead of by errors in our star catalogs.
Conversion table for MPEC epoch dates
The Minor Planet Center has recently begun distributing "daily orbit updates", with each orbital set given on a single 80-character string. This is indeed convenient, although considerable angular precision had to be sacrificed. Also, the epoch of the elements has been stuffed into five cryptic characters, such as "J97CI"; the meaning isn't intuitively obvious.
The format works as follows. 'J' indicates the 1900s. 'K' will eventually indicate the 2000s, and 'I' could be used for historical 1800s orbits. The year is indicated by two digits. The remaining two letters give a month and day; in the above example, 'C' is the month, 'I' the day. They are encoded in a sort of expanded hexadecimal, where C=12 (as in normal hex) and I=18. So the date in question is 18 December 1997 = JD 2450800.5.
MPC seems to be fond of 20-day spacings, so the following table ought to help for a while. (Thanks to Jim Roe, for bringing this subject up.)
JD 2450800.5 = 1997 Dec 18 = J97CI JD 2450820.5 = 1998 Jan 7 = J9817 JD 2450840.5 = 1998 Jan 27 = J981R JD 2450860.5 = 1998 Feb 16 = J982G JD 2450880.5 = 1998 Mar 8 = J9838 JD 2450900.5 = 1998 Mar 28 = J983S JD 2450920.5 = 1998 Apr 17 = J984H JD 2450940.5 = 1998 May 7 = J9857 JD 2450960.5 = 1998 May 27 = J985R JD 2450980.5 = 1998 Jun 16 = J986G JD 2451000.5 = 1998 Jul 6 = J9876 JD 2451020.5 = 1998 Jul 26 = J987Q JD 2451040.5 = 1998 Aug 15 = J988F JD 2451060.5 = 1998 Sep 4 = J9894 JD 2451080.5 = 1998 Sep 24 = J989O JD 2451100.5 = 1998 Oct 14 = J98AE JD 2451120.5 = 1998 Nov 3 = J98B3 JD 2451140.5 = 1998 Nov 23 = J98BN JD 2451160.5 = 1998 Dec 13 = J98CD JD 2451180.5 = 1999 Jan 2 = J9912 JD 2451200.5 = 1999 Jan 22 = J991M JD 2451220.5 = 1999 Feb 11 = J992B JD 2451240.5 = 1999 Mar 3 = J9933 JD 2451260.5 = 1999 Mar 23 = J993N JD 2451280.5 = 1999 Apr 12 = J994C JD 2451300.5 = 1999 May 2 = J9952 JD 2451320.5 = 1999 May 22 = J995M JD 2451340.5 = 1999 Jun 11 = J996B JD 2451360.5 = 1999 Jul 1 = J9971 JD 2451380.5 = 1999 Jul 21 = J997L JD 2451400.5 = 1999 Aug 10 = J998A JD 2451420.5 = 1999 Aug 30 = J998U JD 2451440.5 = 1999 Sep 19 = J999J JD 2451460.5 = 1999 Oct 9 = J99A9 JD 2451480.5 = 1999 Oct 29 = J99AT JD 2451500.5 = 1999 Nov 18 = J99BI JD 2451520.5 = 1999 Dec 8 = J99C8 JD 2451540.5 = 1999 Dec 28 = J99CS JD 2451560.5 = 2000 Jan 17 = K001H JD 2451580.5 = 2000 Feb 6 = K0026 JD 2451600.5 = 2000 Feb 26 = K002Q JD 2451620.5 = 2000 Mar 17 = K003H JD 2451640.5 = 2000 Apr 6 = K0046 JD 2451660.5 = 2000 Apr 26 = K004Q JD 2451680.5 = 2000 May 16 = K005G JD 2451700.5 = 2000 Jun 5 = K0065 JD 2451720.5 = 2000 Jun 25 = K006P JD 2451740.5 = 2000 Jul 15 = K007F JD 2451760.5 = 2000 Aug 4 = K0084 JD 2451780.5 = 2000 Aug 24 = K008O JD 2451800.5 = 2000 Sep 13 = K009D JD 2451820.5 = 2000 Oct 3 = K00A3 JD 2451840.5 = 2000 Oct 23 = K00AN JD 2451860.5 = 2000 Nov 12 = K00BC JD 2451880.5 = 2000 Dec 2 = K00C2 JD 2451900.5 = 2000 Dec 22 = K00CM JD 2451920.5 = 2001 Jan 11 = K011B JD 2451940.5 = 2001 Jan 31 = K011V JD 2451960.5 = 2001 Feb 20 = K012K JD 2451980.5 = 2001 Mar 12 = K013C JD 2452000.5 = 2001 Apr 1 = K0141 JD 2452020.5 = 2001 Apr 21 = K014L JD 2452040.5 = 2001 May 11 = K015B JD 2452060.5 = 2001 May 31 = K015V JD 2452080.5 = 2001 Jun 20 = K016K JD 2452100.5 = 2001 Jul 10 = K017A JD 2452120.5 = 2001 Jul 30 = K017U JD 2452140.5 = 2001 Aug 19 = K018J JD 2452160.5 = 2001 Sep 8 = K0198 JD 2452180.5 = 2001 Sep 28 = K019S JD 2452200.5 = 2001 Oct 18 = K01AI JD 2452220.5 = 2001 Nov 7 = K01B7 JD 2452240.5 = 2001 Nov 27 = K01BR JD 2452260.5 = 2001 Dec 17 = K01CH JD 2452280.5 = 2002 Jan 6 = K0216 JD 2452300.5 = 2002 Jan 26 = K021Q JD 2452320.5 = 2002 Feb 15 = K022F JD 2452340.5 = 2002 Mar 7 = K0237 JD 2452360.5 = 2002 Mar 27 = K023R JD 2452380.5 = 2002 Apr 16 = K024G