The black-drop effect bedeviled attempts to determine the Astronomical Unit from the time of the transit of Venus of 1761, until dynamical determinations of the AU obviated the need for transit measurements. By studying the 1999 transit of Mercury, using observations taken from space with NASA's Transition Region and Coronal Explorer (TRACE), we have fully explained Mercury's black-drop effect, with contributions from not only the telescope's point-spread function but also the solar limb darkening. Since Mercury has no atmosphere, we have thus verified the previous understanding, often overlooked, that the black-drop effect does not necessarily correspond to the detection of an atmosphere. We continued our studies with observations of the 2004 transit of Venus with the TRACE spacecraft in orbit and with ground-based imagery from Thessaloniki, Greece. We report on preliminary reduction of those data; see http://www.transitofvenus.info for updated results. Such studies are expected to contribute to the understanding of transits of exoplanets. Though the determination of the Astronomical Unit from studies of transit of Venus has been undertaken only rarely, it was for centuries expected to be the best method. The recent 8 June 2004 transit of Venus provided an exceptionally rare opportunity to study such a transit and to determine how modern studies can explain the limitations of the historical observations.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We are currently developing an analytical theory of an artificial satellite of the Moon. It is an interesting problem because the dynamics of a lunar orbiter is quite different from that of an artificial satellite of the Earth, by at least two aspects: the $J_2$ lunar gravity term is only $1/10$ of the $C_{22}$ term and the third body effect of the Earth on the lunar satellite is much larger than the effect of the Moon on a terrestrial satellite. So we have to account at least for these larger perturbations. We use here the method of the Lie Transform as perturbation method. The Hamiltonian of the problem is first averaged over the fast angle, in canonical variables. The solution is developed in powers of the small factors linked to $ n_{\Moon}$, $J_2$, $C_{22}$ and to the Earth's position. The Earth location is determined by the lunar theory ELP2000 (Chapront-Touzé & Chapront 1991) from which we take the leading terms. Series developments are made with our home-made Algebraic Manipulator, the MM (standing for “Moon's series Manipulator”). The results are obtained in a closed form, without any series developments in eccentricity or inclination. So the solution applies for a wide range of values, except for few isolated critical values. We Achieved, among others, second order results for the combined effect of $J_2$ and $C_{22}$. As a side result, we were able to check the second order generator $\mathcal{W}_{2}$ given by Kozai for the effect of the $J_2$ term on an artificial satellite.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

One of the main characteristics of Mercury is its 3:2 spin-orbit resonance, combined with a 1:1 resonance between the orbital node of its orbit and the angle describing the precession of the rotation axis, both measured on the ecliptic plane. We build an analytical model, using Hamiltonian formalism, that takes into account this phenomenon thanks to the introduction of three resonant variables and conjugated momenta. We calculate the equilibria corresponding to four different configurations, which means four completely different values of the (ecliptic) obliquity; in particular, we focus on the present (stable) situation of Mercury, and thanks to several canonical transformations, we obtain, near the equilibrium, three pairs of angle-action variables, and consequently, three basic frequencies. Let us note that the model is as simple as possible: the gravitational potential is limited to the second degree terms (the only ones for which a value can be presently given), and the orbit of Mercury is Keplerian. The numerical values obtained by our simplified model are validated by the coherence with existing complete numerical models.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Mutual event observations started in the early 1970s with the Galilean satellites. These observations were needed because of the Voyager spacecraft future arrival. Since 1979, IMCCE has organized observational campaigns for the Galilean satellites (called PHEMU), and since 1995 for the Saturnian satellites (also called PHESAT). Meanwhile, the reduction techniques have been greatly improved. Mutual event observations are one of the most accurate methods for obtaining positions of natural satellites, useful for detecting tidal effects. Hence mutual events of Jovian and Saturnian natural satellites are regularly observed around the world. This paper aims to describe mutual events and the advantages of this kind of observation besides the classical astrometric ones.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

The orbital distribution of asteroids and Kuiper-belt objects (KBOs) provides important information for the dynamical evolution of the solar system. Recent advances in modelling the dynamics of asteroids and KBOs have increased our understanding of the mechanisms responsible for the main features observed in the small-body belts. There are, however, several pieces left to complete the puzzle of the dynamical history of the solar system. In particular, we now understand that the solar system probably looked very different during the first $500-1000$ Myr than today, mainly because of two processes that took place during those times: (i) planetary formation and (ii) planetary migration. I will discuss observational and dynamical constraints that any model, attempting to reconstruct this early period, should obey. I will then present a new model for planetary migration, which successfully reproduces the observed orbital distribution of the trans-Neptunian objects. I will then discuss the implications of this model on the early evolution of the inner solar system, in particular the distribution of main-belt asteroids and the bombardment of the terrestrial planets by small bodies.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

With the substantial improvements in observational techniques we have to deal with very big databases, consisting of a few positions of an object over a short time span; this is often not enough to compute a preliminary orbit with traditional tools. In this paper we first review a classical method by C.F. Gauss to compute a preliminary orbit for asteroids. This method, followed by a least squares fit to improve the orbit, still today gives successful results when we have at least three separate observations. Then we introduce the basics of a very recent orbit determination theory, that has been thought just to be used with modern sets of data. These data allow us in many cases to know the angular position and velocity of an asteroid at a given time, even though the radial distance and velocity $(r,\dot r)$, needed to compute its full orbit, are unknown. The variables $(r,\dot r)$ can be constrained to a compact set, that we call the admissible region(AR), whose definition requires that the body belongs to the Solar System, that it is not a satellite of the Earth, and that it is not a “shooting star” (i.e. very close and very small). We provide a mathematical description of the AR: its topological properties are surprisingly simple, in fact it turns out that the AR cannot have more than two connected components. A sampling of the AR can be performed by means of a Delaunay triangulation; a finite number of six-parameter sets of initial conditions are thus defined, with each node of the triangulation representing a possible orbit (a virtual asteroid).To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Transits of Venus occur in pairs 8 yr apart, the pairs separated by intervals of either 112 or 130 yr, because of the pattern of approximate orbital resonances. Transits of Mercury show a different pattern, partly because of the more eccentric orbit, and also because the structure of the approximate resonances is different. The near-resonance in Venus' axial rotation strongly suggests a tidal link with the Earth.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

This presentation has the following goals: (i) to explain, within a historical context, some of the difficulties in disentangling our Galaxy's three-dimensional structure; (ii) to summarise in broad terms what we presently know of this structure, concentrating on those features inferred from accurate distance measurements; (iii) to illustrate selected aspects in a visually stimulating manner; and (iv) to give a foretaste of the exciting results that lie ahead with future space astrometric experiments. The public lecture given in Preston, UK on 9 June 2004 employed 3-d visualisation techniques using polarised light to illustrate the talk.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

During the 25 years that the author has been involved in astrometry, the quality of ground-based parallaxes has increased by about a factor of 10 (from 3 mas to 0.3 mas), but the quantity has increased by only a few hundred. When asked, the average astronomer will cite the $H^2$ space missions (Hubble and Hipparcos) as the great advances in astrometry even thought the ground-based work has played a critical role in the understanding classes of stars such as the L- and T-dwarfs. The next decade promises stunning advances in both the quality and quantity of parallax measurements, and both ground- and space-based projects will play significant roles. The Gaia space mission and the various ground-based telescopes with large etendue (DMT, LSST, Pan-STARRS, etc.) will improve the quality or quantity (or both) by factors of a thousand or more. The situation will be discussed, and the author will express his hope that he might live long enough to see the fruits of these labors.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We describe our experiences with on-orbit calibration of, and scientific observations with, the Fine Guidance Sensors (FGS), white-light interferometers aboard Hubble Space Telescope. Our original goal, 1 milliarcsecond precision parallaxes, has been exceeded on average by a factor of three, despite a mechanically noisy on-orbit environment, the necessary self-calibration of the FGS, and significant temporal changes in our instruments. To obtain accurate absolute parallaxes from these small fields of view ($3^\prime \times 15^\prime$) observations requires a significant amount of ancillary reference star information. These data also permit an independent estimate of interstellar absorption, critical in determining target absolute magnitudes, M$_V$, often the key result of a parallax program. With these techniques we and our collaborators have obtained absolute parallaxes for 21 astrophysically interesting objects. We briefly discuss a recent determination of the parallax of the Pleiades. HST routinely produces parallaxes with half the error of the best Hipparcos results, a precision that continues down to target $V = 15$. The FGS will remain a competitive astrometric tool for the generation of high-precision parallaxes until the advent of longer-baseline space-based interferometers (SIM), or the failure of some key HST component.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Determinations and measurements of the parallax of the Pleiades, obtained with ground-based studies and with Hipparcos data, are reviewed. A number of uncertainties in both sets of data are found. Although a further correction for abscissa correlations brings the Hipparcos determination closer to the ground-based value, the difference still seems too large. The new Hipparcos parallax determinations seem to reinforce a possible age related effect. A new reduction of the Hipparcos data is in progress. It reduces significantly the contribution of the attitude noise, providing higher accuracies and lower correlation levels for the brighter stars.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

The distance to the Pleiades open cluster is of fundamental importance to many aspects of stellar astrophysics but is currently controversial. The ‘long’ distance scale of $132 \pm 3$ pc is supported by main sequence fitting analyses, ground-based parallax observations and analysis of the astrometric binary Atlas. The ‘short’ distance scale of $120 \pm 3$ pc comes from parallaxes observed by the Hipparcos satellite. Munari et al. (2004) studied the detached eclipsing binary HD 23642 and found a distance of $132 \pm 2$ pc. We reanalyse the data of Munari et al. to explore the different methods of estimating the distance of an eclipsing binary system. We use the surface brightness relations of Kervella et al. (2004) to find a distance of $139 \pm 4$ pc, which is consistent with the ‘long’ Pleiades distance and in disagreement with the Hipparcos parallax distances to the Pleiades and to HD 23642 itself. By comparing the observed masses and radii of the binary with theoretical predictions we derive a metal abundance approximately equal to or slightly greater than solar. Further photometric observations of the binary are needed to improve the analysis.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

The change in the measured position due to the chromatic effect in the optical system is usually so small that it can be neglected. However, in the case of the Hipparcos mission, this effect is of the same magnitude as the targeted precision; it is therefore mandatory to correct for it. The different assumptions adopted to build up such corrections in the Hipparcos framework are presented. Some of their limitations and how they can be overcome to improve the results even today are described. The chromaticity mainly affects red stars and has its strongest impact on the astrometry of very red and highly variable stars such as Miras. Nevertheless, the implications on distance scale determination are rather small.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Eclipsing binary light curves have provided the ‘royal road’ (Russell 1948) to fundamental astrophysical information on stars. If radial velocities of the components and a reliable colour/temperature/flux relation are available, parallaxes may be determined and compared with direct measures, e.g. by Hipparcos. Accuracies of existing measures are considered and aspects of the development of this and other methods of distance determination reviewed. Roles for multiwavelength techniques (e.g. VLBA, broad-spectrum photometry) are noted. The recovery of information on new planets orbiting remote stars by transit phenomena will be looked into within this context.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Very long baseline interferometry (VLBI) is a radio astronomy technique that allows imaging and astrometry at milliarcsecond or better resolution. The last decade has seen improvements in VLBI techniques and equipment which have enabled precision astrometry of pulsars. Most notably, VLBI arrays have become more sensitive and calibration has become more accurate. Pulsar astrometry using VLBI can readily yield a proper motion and in many cases a trigonometric parallax as well. These observables have have numerous applications which justify the difficulty of the measurements. The goals of pulsar astrometry include: 1. determination of the distribution of pulsar birth velocities, a critical probe of supernova asymmetry; 2. improvement in understanding of the ISM through observations of pulsar bow shocks, interstellar scattering, and timing dispersion measures; and 3. understanding characteristics of individual pulsars though knowledge of their distance and velocity. I will discuss these science cases, some recent results, and prospects for better astrometry in the near future.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We examine statistical methods for calibrating trigonometric parallaxes to retrieve the absolute magnitudes of stars, using Monte Carlo simulations. Here we consider the case of the zero-point of the period-luminosity relation for Cepheid variables. The method originally proposed by Ratnatunga & Casertano was revisited by introducing a realistic density distribution of sample stars belonging to the catalogue through prior calculations of the photometric distance for each star. It is found that our method gives an unbiased estimate, regardless of any dispersions in their absolute magnitude. We further investigate the reliability of results which depend on the accuracy of parallax. Our finding is that the accuracy ($\sim$1 mas) of Hipparcos parallaxes is not enough to obtain a reliable result due to a large variation among different ensembles of stars. More precise determination of parallaxes to an accuracy of 200 $\mu$as at least, which will be easily realized by the ongoing astrometric space satellites, will give a precise zero-point together with a dispersion in absolute magnitude.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We discuss a new program to measure the parallaxes of a number of L and T dwarfs, objects that bridge the gap between M dwarfs and planets. This pilot project tests the feasibility of using large telescopes with infrared detectors to determine parallaxes at the level of milli-arcseconds (mas). First results show that we are able to achieve the required centroiding precision and simulations indicate that when the final observations come in we should be able to achieve our goal of parallaxes with accuracies of 2 mas. The main problems will be focal plane astrometric distortions and stability.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Gaia is an all-sky, high precision astrometric and photometric satellite of the European Space Agency (ESA) due for launch in 2010. Its primary mission is to study the composition, formation and evolution of our Galaxy. Over the course of its five-year mission, Gaia will measure parallaxes and proper motions of every object in the sky brighter than visual magnitude 20, amounting to a billion stars, galaxies, quasars and solar system objects. It will achieve an astrometric accuracy of 10 $\mu$as at $V=15$ – corresponding to a distance accuracy of 1% at 1 kpc – and 200 $\mu$as at $V=20$. With Gaia, tens of millions of stars will have their distances measured to a few percent or better. This is an improvement over Hipparcos by several orders of magnitude in the number of objects, accuracy and limiting magnitude. Gaia will also be equipped with a radial velocity spectrograph, thus providing six-dimensional phase space information for sources brighter than $V$$\sim$ 17. To characterize the objects (which are detected in real time, thus dispensing with the need for an input catalogue), each object is observed in 15 medium and broad photometric bands with an onboard CCD camera. With these capabilities, Gaia will make significant advances in a wide range of astrophysical topics. In addition to producing a detailed kinematical map of stellar populations across our Galaxy, Gaia will also study stellar structure and evolution, discover and characterise thousands of exoplanetary systems (extending down to about ten Earth masses for the nearest systems) and make accurate tests of General Relativity on large scales, to mention just some areas. I give an overview of the mission, its operating principles and its expected scientific contributions. For the latter I provide a quick look in five areas on increasing scale size in the universe: the solar system, exosolar planets, stellar clusters and associations, Galactic structure and extragalactic astronomy.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Gaia is a mission to map the positions and determine the velocities of 1 billion stars in the Galaxy, in order to determine how it was formed and how it has evolved. Spectroscopy provides one component of the space velocities for 200 million objects, and, in addition, astrophysical information such as temperature and metallicity for the brighter 40 million stars in a large unbiased Galactic sample. Together the instruments on Gaia will transform our knowledge of the Galaxy. We describe here a sample of the science that will be done with Gaia concentrating on aspects in which the spectroscopy plays a central role.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We introduce a Japanese plan for infrared (z-band: 0.9 $\mu$m) space astrometry (the JASMINE-project). It will measure parallaxes, positions with the accuracy of 10 $\mu$as and proper motions with the accuracy of 10 $\mu$as/yr for stars brighter than z$\sim$14. JASMINE can observe about $10^8$ stars belonging to the disk and bulge components of our Galaxy which are hidden by interstellar dust extinction in optical bands. The number of stars with $\sigma_{\pi}/\pi<0.1$ in the direction of the Galactic central bulge is about $10^3$ times larger than those observed in optical bands, where $\pi$ is a parallax and $\sigma_{\pi}$ is an error of the parallax. The main objective of JASMINE is to provide very useful and important astrometric parameters for studying fundamental structures and evolution of the disk and bulge components of the Milky Way Galaxy. Furthermore, the astrometric parameters given by JASMINE will give us exact absolute luminosities and motions of many stars in the bulge and the disk far away from us, so it will promote the study of stellar physics. The information of infrared astrometry that JASMINE will provide is very useful also for investigating stars in star formation regions, gravitational lens effects due to disk stars, extra-solar planets, etc. JASMINE will be launched around 2014 and a candidate for the orbit is a Lissajous orbit around the Sun-Earth L2 point with about a 5-yr mission life. We adopt a 3-mirror optical system (modified Korsch system) with a primary mirror of $\sim$1.5-m diameter in an instrument design of JASMINE. A beam combiner should be used for performance of the global astrometry as used in the Hipparcos satellite. On the astro-focal plane, we put about 100 new-type CCDs for the z-band in which TDI mode (drift scan mode) can be operated. The effective field of view is 0.23 square degrees. The consideration of overall system (bus) design is now going on in cooperation with the Japan Aerospace Exploration Agency (JAXA). Furthermore, we introduce the Nano-JASMINE project which uses a nano-satellite with a size of about 20 cm3 and a weight of a few kg. The objective of Nano-JASMINE is verification of the observing strategy adopted in JASMINE and examination of some important technical issues for the JASMINE project. It will be launched around 2006.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html