Bright star Deneb transits at nightfall

Each year around October 24, the northernmost star of the famous Summer Triangle, Deneb, transits – climbs to its highest point in the sky – around 6:30 p.m. local time (7:30 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 each day in our sky as Earth travels around the sun. Its transit at dusk and nightfall is a hallmark of the year, marking a shift toward winter (or summer) on your half of the globe.

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 45 degrees north latitude (St. Paul, Minnesota, and Turin, Italy), Deneb shines straight overhead when it transits.

At 40 degrees north latitude (Denver, Colorado, and Beijing, China), Deneb soars to its highest point (about 5 degrees north of zenith) as evening dusk is giveing way to nightfall.

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Celestial sphere with meridian and horizon at right angles, and axis at a slanted angle.
The meridian is the imaginary semicircle that arcs across the sky from due north to due south. The sun or any star climbs to its highest point for the day when it crosses your meridian.

At temperate latitudes in the Southern Hemisphere, where it’s now springtime, Deneb transits at or near the same hour by the clock (near 6:30 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 when the sun is still up, instead of at dusk/nightfall.

At more northerly or southerly latitudes, Deneb either transits to the north or to the south of the zenith point. Appreciably south of 45 degrees north latitude, Deneb lies to the north of the zenith point when it transits; conversely, when Deneb transits at latitudes appreciably north of 45 degrees 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.

Star field with cloudy Milky Way running across it, and stars and star groups labeled.
The Great Rift of the Milky Way passes through the constellation Cassiopeia and the Summer Triangle. Click here for a larger photo.

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 four minutes earlier with each following day (or two 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 in mid-October, the star Deneb shines at the apex of the sky at mid-northern latitudes.

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October 24, 2020

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Bruce McClure

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