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In observational astronomy, an observation arc (or arc length) is the time period between the first and most recent (last) observation, tracing the body's path. It is usually given in days or years. The term is mostly used in the discovery and tracking of asteroids and comets.
The observation arc determines how accurately known the orbit of the object is. A very short arc could describe objects in a wide variety of orbits, at many distances from Earth. In some cases, there have been objects whose initial arc was insufficient to determine if the object was in orbit around the Earth, or orbiting out in the asteroid belt. With a 1-day observation arc, 2004 PR107 was thought to be a trans-Neptunian dwarf planet, but is now known to be a 1 km main-belt asteroid. With an observation arc of 3 days 2004 BX159 was thought to be a Mars-crossing asteroid that could be a threat to Earth, but it is now known to be a main-belt asteroid.
An observation arc less than 30 days can make it difficult to recover an Inner Solar System object more than a year after the last observation, and may result in a lost minor planet. Due to their greater distance from the Sun and slow movement across the sky, trans-Neptunian objects with observation arcs less than several years often have poorly constrained orbits.