{"group":"place","pic":"living/pics/sky-city.webp","question":"What is the sky like over Arcadia?","answer":"Butterscotch by day and blue at sunset, with a smaller Sun, two moons running opposite ways and, in its season, Earth shining as the evening star.","play":{"label":"Watch the sky change","href":"play/?era=city&light=dusk"},"moons":{"title":"Two moons, two directions","line":"Facing south over the plain at night.","east":"East","west":"West","south":"South","phobos":"Phobos","deimos":"Deimos","ph_text":"Phobos circles Mars in {ph_h}, faster than Mars turns, so it rises in the west and races east.","de_text":"Deimos takes {de_h} to go round, barely faster than Mars turns, so it creeps from east to west over days.","speed":"Speed up the night"},"lives_title":"How the city lives by the sky","lives":[{"icon":"house","name":"Rooms that face the sunset","text":"Viewing rooms look west and south: towards the sunset, the evening star and Phobos's path."},{"icon":"bulb","name":"Lights kept low","text":"The town lives under its soil and keeps its outdoor lights low, so the stars blaze over the plain."},{"icon":"signal","name":"Earth, the evening star","text":"When Earth returns as the evening star after hiding behind the Sun, messages from home return with it."},{"icon":"sun","name":"Sunset, the event of the day","text":"The blue sunset is the sight everyone stops for, and its glow lingers for up to {tw_h} hours."}],"blue":{"title":"Why the sunset turns blue","line":"Fine dust, about {dust_d} µm across, hangs in the air all the time.","sun":"Sun","eye":"You","dust":"Dust in the air","fwd":"Blue goes forward","wide":"Red spreads wide","text":"Dust grains this size send blue light onward, close to the Sun's direction, and spread the warm colours across the whole sky. So the day sky glows butterscotch, and at sunset, looking towards the Sun, you see blue."},"suns":{"title":"A smaller, gentler Sun","line":"Both discs drawn to the same scale.","earth":"From Earth","mars":"From Mars","text":"From Mars the Sun looks {sun_pct}% as wide as from Earth and gives {f_sun_pct}% of the light: still a blinding disc, in a sky the colour of honey.","unit":"arcminutes"},"earth_mars":{"earth":{"head":"A blue sky and a red sunset"},"mars":{"head":"A butterscotch sky and a blue sunset"},"pics":{"earth":"living/pics/em-sky-earth.webp","mars":"living/pics/em-sky-mars.webp"},"rows":[{"label":"By day","icon":"sun","earth":"Blue overhead, from the air itself scattering blue light.","mars":"Butterscotch overhead, from fine dust hanging in the air."},{"label":"At sunset","icon":"wave","earth":"Reds and oranges gather round the setting Sun.","mars":"A cool blue glow wraps the Sun and lingers for up to {tw_h} hours."},{"label":"Moons","icon":"mars","earth":"One Moon, rising in the east like the Sun.","mars":"Two: Phobos races west to east, Deimos creeps east to west."}]},"living":"The day ends with the best show on Mars: the blue sunset. Then the stars come out by the thousand, Phobos climbs out of the west, and in its season Earth shines above the glow as the evening star.","fact":"Perseverance has filmed Phobos crossing the face of the Sun from Jezero again and again, most recently on {ph26_date}. Each crossing is over fast: the one on {ecl_date} took a little over {ecl_s} seconds.","later":{"pics":{"city":"living/pics/sky-city.webp","far":"living/pics/sky-far.webp"},"city":{"tag":"Viewing rooms","text":"People watch the sky from viewing rooms, periscopes and evening walks in their suits."},"far":{"tag":"Sky overhead","text":"Under the domes' water roofs the sky is there all day, softened to a shimmering blue."}},"pin":[0.593,0.5],"scene":{"pic":"living/pics/scene-sky.webp","look":"02-07","shot":"blue_sunset_room","focus":"60% 45%","spots":[{"at":[0.73,0.55]},{"at":[0.13,0.43]},{"at":[0.655,0.24],"zoom":2.6},{"at":[0.6,0.47]}]},"proof":{"pic":"living/people/sky-proof.webp","place":"Jezero Crater, Mars","when":"{ph26_date}","head":"A moon crossing the Sun","credit":"Photo: NASA/JPL-Caltech/ASU/MSSS/SSI (public domain)"},"quiz":[{"q":"What colour is the glow round the setting Sun on Mars?","a":["Blue","Orange","Green"],"why":"Fine dust sends blue light onward, close to the Sun's direction, so the glow round the setting Sun is blue.","more":"The first colour picture from the surface, taken on {viking_date}, was first shown with a blue sky. Properly calibrated, the sky was reddish.","cite":"02-07 F-01, F-02","from":"why","fig":0,"icon":"sun"},{"q":"Which way does Phobos cross the sky?","a":["West to east","East to west","It hangs still"],"why":"Phobos circles Mars faster than Mars turns, so it rises in the west and sets in the east.","more":"Phobos is slowly falling: Mars draws it about {ph_fall} m closer every hundred years.","cite":"02-07 F-07, F-12","from":"problem","fig":1,"icon":"mars"},{"q":"How wide does the Sun look from Mars?","a":["{sun_pct}% as wide as from Earth","Twice as wide","Exactly the same"],"why":"From Mars the Sun is about {sun_m} arcminutes across, against {sun_e} from Earth.","more":"Now and then Earth crosses the face of the Sun as seen from Mars. The next time is on {tr_next}.","cite":"02-07 F-05, F-19","from":"why","fig":1,"icon":"target"}],"notes":[{"beat":"fix","text":"Mars has green auroras too. On {au_date} Perseverance saw one: the first aurora ever seen in visible light from the ground of another planet.","cite":"02-07 F-26"}],"from":"From the viewing room","everyday":{"title":"A day under the Mars sky","items":[{"time":"06:20","head":"A long blue dawn","text":"The glow starts climbing in the east up to {tw_h} hours before sunrise, as dust high in the air catches the light.","tag":"{tw_h} h of twilight","pic":"living/pics/plain-city.webp","pos":"78% 15%"},{"time":"12:00","head":"A gentle noon","text":"The Sun stands small and bright in a butterscotch sky, warm on your visor and easy on the eyes of every camera.","tag":"{sun_pct}% as wide","pic":"living/pics/em-dangers-mars.webp","pos":"70% 10%"},{"time":"21:30","head":"Phobos rises in the west","text":"The little moon climbs out of the west and slides east, low over the southern sky.","tag":"Round Mars in {ph_h}","pic":"living/pics/sky-city.webp","pos":"38% 52%"}]}}