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Context.  The second solution of the  Gaia  catalog, which has been available since April 2018, plays an important role in the realization of the future  Gaia  reference frame. Since 1997, the reference frame has been materialized by the optical H IPPARCOS  positions of about 120 000 stars. The H IPPARCOS  has been compared with and linked to the International Celestial Reference Frame (ICRF). The ICRF is materialized by means of the radio positions of extragalactic sources using very large baseline interferometry observations. Both, the H IPPARCOS  and  Gaia  missions belong to the European Space Agency, and it is important to note that the  Gaia  catalog is going to replace the H IPPARCOS  catalog.   Aims.  It has been shown that the International Latitude Service zenith telescope data pertaining to ground-based surveys that span a time baseline of about 80 yr, and which are also key when measuring proper motions, could be useful for the accurate determination of  μ   δ   for 387 ILS stars. Therefore, in this study we aim first to reduce these stars to the H IPPARCOS  reference system; second, to made our original catalog of  μ   δ  , which we refer to as the ILS catalog, for these 387 bright stars; third, to present comparison results of the four catalogs by pairs (the ILS, H IPPARCOS  or HIP, new H IPPARCOS  or NHIP, and  Gaia  DR2); and fourth, to analyze the differences in  μ   δ   between pairs of catalogs to characterize the  μ   δ   errors for these catalogs with a special focus on the  Gaia  DR2 and ILS catalogs.   Methods.  At seven ILS sites around the world at latitude 39 .  ° 1, a set of seven telescopes was used to monitor the latitude variation via observations of the same stars for about 80 yr. Here, the inverse task was applied to improve  μ   δ   values of the 387 H IPPARCOS  stars using the previously mentioned observations. Due to the specific Horrebow-Talcott method of the measured star pair, it is difficult to determine  μ   δ   for each single star. However, we achieved this by developing the original method and in combination with the H IPPARCOS  data. We used the previously developed least squares method and formula to determine the coefficients, which describe the systematic part of differences in  μ   δ   between the pairs of catalogs.   Results.  We calculated the coefficients with the aforementioned formula (in line with the coordinates, stellar magnitude, and color index of every star) to compare ILS, HIP, NHIP, and  Gaia  DR2 data of  μ   δ   against each other by using the set of 387 stars. The presented differences of  μ   δ   show that the systematic errors in the four catalogs are nearly at the same level of 0.1 mas yr −1 . This means that the DR2 and ILS  μ   δ   values are in good agreement with each other, and with values from the H IPPARCOS  and new H IPPARCOS  catalogs. Also, the random errors of differences are small ones; they are near 1 mas yr −1  for ILS-HIP and ILS-NHIP, and about 2 mas yr −1  for ILS-DR2, HIP-DR2, and NHIP-DR2. It is important to note that there is a similar level of proper motion formal errors in H IPPARCOS  and new H IPPARCOS  catalogs.

Comparison of proper motions in declination for 387 Gaia DR2 and HIPPARCOS stars from ILS observations over many decades