  The Ariel mission will observe spectroscopically around 1000 exoplanets to
further characterise their atmospheres. For the mission to be as efficient as
possible, a good knowledge of the planets' ephemerides is needed before its
launch in 2028. While ephemerides for some planets are being refined on a
per-case basis, an organised effort to collectively verify or update them when
necessary does not exist. In this study, we introduce the ExoClock project, an
open, integrated and interactive platform with the purpose of producing a
confirmed list of ephemerides for the planets that will be observed by Ariel.
The project has been developed in a manner to make the best use of all
available resources: observations reported in the literature, observations from
space instruments and, mainly, observations from ground-based telescopes,
including both professional and amateur observatories. To facilitate
inexperienced observers and at the same time achieve homogeneity in the
results, we created data collection and validation protocols, educational
material and easy to use interfaces, open to everyone. ExoClock was launched in
September 2019 and now counts over 140 participants from more than 15 countries
around the world. In this release, we report the results of observations
obtained until the 15h of April 2020 for 119 Ariel candidate targets. In total,
632 observations were used to either verify or update the ephemerides of 83
planets. Additionally, we developed the Exoplanet Characterisation Catalogue
(ECC), a catalogue built in a consistent way to assist the ephemeris refinement
process. So far, the collaborative open framework of the ExoClock project has
proven to be highly efficient in coordinating scientific efforts involving
diverse audiences. Therefore, we believe that it is a paradigm that can be
applied in the future for other research purposes, too.
