Tonight – October 18, 2017 – the northernmost star of the Summer Triangle, Deneb, transits or climbs to its highest point in the sky at or near 7 p.m. local time (8 p.m. local Daylight Saving Time). What does that mean for skywatchers? Only that this noteworthy star – this beloved member of the Summer Triangle – is shifting ever-westward in our sky as Earth travels around the sun. Its transit at nightfall might be considered a kind of hallmark of the year, marking a shift toward winter – or summer – depending on whether you’re in the Northern or Southern Hemisphere.
When the sun or a star transits, it resides at one of three places: at zenith (straight overhead), north of zenith or south of zenith.
At 45o north latitude (St Paul, Minnesota, or Turin, Italy), Deneb shines straight overhead when it transits.
At 40o north latitude (the latitude of Denver, Colorado or Philadelphia, Pennsylvania), Deneb soars to its highest point about one and one-half hours after sunset, or as evening dusk gives way to nightfall.
At temperate latitudes in the Southern Hemisphere, where it’s now springtime, Deneb transits at or near the same hour by the clock (near 7 p.m. local time). Yet, the sun sets later by the clock at more southerly latitudes, so in the Southern Hemisphere, Deneb at this time of year actually transits at evening dusk, instead of nightfall.
At more northerly or southerly latitudes, Deneb either transits to the north or to the south of the zenith point. Appreciably south of 45o north latitude, Deneb lies to the north of the zenith point when it transits; conversely, when Deneb transits at latitudes appreciably north of 45o north latitude, Deneb is viewed in the southern sky.
Two brilliant stars – Vega and Altair – team up with Deneb to complete the humongous Summer Triangle. The luminous Summer Triangle asterism, or star formation, can often be seen in a twilight sky or even from a light-polluted city.
From mid-northern latitudes, the far-northern stars Deneb and Vega are seen at the “top” of the Summer Triangle whereas the southernmost star Altair is seen at the “bottom.” From the Southern Hemisphere, it’s the other way around: Altair reigns at top and Deneb at bottom. It’s a matter of perspective.
Vega, the Summer Triangle’s westernmost star, is seen to the right of Deneb from mid-northern latitudes. From the Southern Hemisphere, on the other hand, Vega lies to the left of Deneb.
Around the world, the stars of the Summer Triangle transit some 4 minutes earlier with each following day (or 2 hours earlier with each following month). So, from northerly latitudes, the Summer Triangle is destined to shift over into the western sky at nightfall as autumn ebbs toward winter … or, for those in the Southern Hemisphere, as spring blooms into summer.
Bottom line: As darkness falls on October 18, the star Deneb shines at the apex of the sky at mid-northern latitudes.