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*To*: orekit-users@orekit.org*Subject*: Re: [Orekit Users] Surface Illumination*From*: MAISONOBE Luc <luc.maisonobe@c-s.fr>*Date*: Tue, 12 Jul 2016 16:04:43 +0200*In-reply-to*: <CAD4DT2h1p2JY3_2e1mWHQzot3o_oAfs2UR8RNHygUTcspyF-Aw@mail.gmail.com>*User-agent*: Internet Messaging Program (IMP) H5 (6.2.3)

Stephen Ranger <sanosuke001@gmail.com> a écrit : Hi Stephen,

Hello Users, Thanks for the info about the inertial frame the other day. Another question as I can't seem to find anything through Google. I'm looking to mark a map based on time of day to show day/night/twilight/etc. Is there any way using Orekit to get the illumination level at a specific point on Earth? If not, any idea where I should start looking?

You will need the following items: - a OneAxisEllipsoid for Earth model - a GeodeticPoint for each point in the map you want to consider - an AbsoluteDate for the current date - a CelestialBody for the Sun, considered as a moving point (use CelestialBodyFactory.getSun() to get it) Then you first compute the Sun position in Earth rotating frame:

For each of your geodetic points, compute its Cartesian coordinates: Vector3D groundPointPos = earth.transform(geodeticPoint); You can now compute the direction from ground point to Sun: Vector3D sunDirection = sunPos.subtract(groundPointPos); And finally the elevation angle of the Sun can be computed as:

physical model. The algorithm above does not consider atmospheric refraction. If you want to

interface (Orekit provides both EarthStandardAtmosphereRefraction and EarthITU453AtmosphereRefraction). The getRefraction() method provides the necessary correction as a function of the elevation angle. Considering refraction is important if you are interested in twilight, because the refraction is important near the horizon. best regards, Luc

Thanks for the help! Stephen

**Follow-Ups**:**Re: [Orekit Users] Surface Illumination***From:*Stephen Ranger <sanosuke001@gmail.com>

**References**:**[Orekit Users] Surface Illumination***From:*Stephen Ranger <sanosuke001@gmail.com>

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