THE SOUTHWORTH PLANETARIUM
70 Falmouth Street      Portland, Maine 04103
(207) 780-4249      usm.maine.edu/planet
43.6667° N    70.2667° W  Founded January 1970
2022-2023: LXXX
Sunrise: 6:20 a.m.
Sunset: 7:10 p.m.
Civil twilight begins: 5:50 a.m.
Civil twilight ends: 7:39 p.m.
Sun's host constellation: Pisces
Moon phase: Waxing gibbous (93% illuminated)
Moonrise: 4:39 p.m.
Moonset: 5:55 a.m. (4/4/2023)
Julian date: 2460037.29
"Keeping a watchful eye on a complex sky"

THE DAILY ASTRONOMER
Monday, April 3, 2023
Our Highest Star

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Part I of the Night Sky Calendar posts tomorrow.
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What is the highest star that appears above us, and why?
<https://www.quora.com/What-is-the-highest-star-that-appears-above-us-and-why>

The answer to that question depends on the observer’s location.

Let’s imagine that Earth is a transparent glass sphere. When one stands on
Earth and looks around one notices stars in every direction: just as many
below the feet as exist above the head.

[Image credit: NASA]

Notice -in your imagination- that every point on this ‘Earth sphere’
corresponds to the position of a star. You stare directly overhead, or at
the zenith, and you’ll see a specific star at the uppermost position for a
moment. As the planet rotates, different stars will occupy the zenith at
specific times. Only those stars whose declinations (angles north or south
of the celestial equator, the projection of Earth’s equator onto the sky)
equal the observer’s latitude will ever see that star pass through the
zenith.

Regard Polaris, the ‘north’ star:

[Image credit: NASA]

As this star is almost aligned precisely with Earth’s north pole, it will
just about appear overhead from the perspective of a north pole observer.*
From this location, at one end of the axis, no other star will appear to
occupy the zenith** throughout the rotational cycle.

Now, regard the star Betelgeuse:

[Image credit: Sky and Telescope]

As its declination is about 7 degrees,* Betelgeuse will appear to pass
directly overhead from the perspective of any observer located along the
7th North parallel.

Finally, let’s look at the night sky’s brightest star, Sirius:

[Image credit: http://Aasimonline.com <http://aasimonline.com/>]

Sirius’ declination is almost -17 degrees.**** Consequently, it will pass
directly overhead along the 17th S parallel. This last statement makes
sense as we can see that Sirius appears south of Betelgeuse and so should
appear directly overhead along a parallel that is south of the 7th north
parallel.

To determine where any given star will pass through the zenith, simply look
up its declination. That star will pass directly overhead along the
latitude which is equal to that declination.

I hope this answer proves helpful.

*With a current declination of 89° 15′ 38.1″ Polaris is not exactly at the
north celestial pole, but is so close that an observer at the North Pole
would be hard pressed to notice the difference.

**Yes, one would find stars closer to the north celestial pole than
Polaris, but let’s cheerfully neglect them because they’re not prominent.

***7° 24′ 26″

****-16° 43′ 06″




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