The following are true and do not change perceptibly over an entire human lifetime.
- The non-sun stars do not move relative to each other. Not now, not over thousands of years, not anytime we will ever know about. The stars remain a fixed spherical image we gaze out at from the inside.
- The north star and southern cross remain in the same positions in the sky and do not move relative to an observer on Earth, even as Earth rotates. They mark the axis of rotation of Earth. Correct, they do not move at all. You could build a structure pointed at the north star one night and if the structure doesn’t move, it points at the north star day and night season after season year after year forever, whether you can see the north star or not, it’s there. (The structure would be called a gnomon, if you care).
- All of the non-sun stars/constellations remain in the exact same latitude and therefore trace the exact same line through the sky each time Earth rotates, every day of our lives, no exceptions. The line a star traces through the sky at your location peaks at 90° – [your latitude] + [latitude of the star] always.
- Earth’s rotational axis is not tilted. Earth’s orbital plane around the sun is tilted! When orienting celestial objects, we are forced to choose what we consider “upright.” Earth’s gravity cannot dictate “upright” for celestial objects. If you imagine Earth’s rotational axis as upright, the north star remains fixed as “north” or “up” and the orbital plane is then tilted 23° meaning Earth “moves north and south” as it orbits, or moves “up and down.”
- The sun. slightly more complicated, but helpful because the sun is bright. What is a line of latitude? What does a line of latitude look like? The blindingly bright sun traces a line of latitude through the sky every day. On equinox days, the sun traces the 0° line of latitude (celestial equator) and on the solstices the sun traces the +/- 23.4° line of latitude. In between, the sun gradually traces lines of latitude from 0° to 23.4° to 0° to -23.4° and back to 0° again throughout the year. Each day the sun traces a slightly different line of latitude as Earth orbits.
- Nobody is good at three-dimensional spherical-angle geometry! Nobody! Astronomers are not good at it. Nobody is! Then why does the current zodiac constellation represent the constellation behind the sun that we cannot see?? Because nobody is good at three dimensional geometry! Astronomers for millennia past and still today use the sun as a “pointer.” Nothing in the sky points like the sun. The sun is such a bright “pointer” that you cannot see what is behind it – but remember that the stars do not move relative to each other so if you have some idea of what the starry sky looks like, and you know the current zodiac constellation, you can reference the rest of the starry sky off of the sun itself. “Pisces is shining bright this month!” True statement – even though Pisces is only up during the day because looking forward the sun means looking toward the current zodiac constellation. We are currently in Pisces. “The sun pointer is moving toward the constellation Aries.” Another true statement. The line the sun traces through the starry background as we orbit is called the “ecliptic.”
Once you have some firm ground to stand on, you can start to remember some more basics and build. If you get confused, re-read the above and remind yourself that many things in the sky do not change!
- Orion, the most widely-recognized constellation, is on the celestial equator. Therefore Orion rises directly east and sets directly west. Orion is “up” ~12 hours and “down” ~12 hours. Orion’s path peaks at 90° – [your latitude] above your horizon (directly overhead the equator). Also, Orion is visible from everywhere on Earth.
- Orion is directly south of the border of Taurus and Gemini. The Taurus Gemini border marks the northern hemisphere summer solstice, so on the summer solstice the sun points near Orion. On the winter solstice, Orion is high in the sky at midnight.
- Polaris, the north star, is located directly north at [your latitude]° above the horizon, always. Most people use the Big Dipper to find Polaris.
- The two stars in the Big Dipper aligned with Polaris point to the ecliptic at Leo near its border with Virgo, so near the sun’s location at the autumnal equinox.
- The Milky Way’s bright galactic center is among Sagittarius, specifically Sagittarius A. It is 29° south of the celestial equator so therefore peaks at 61° – [your latitude] in your sky (more visible in southern hemisphere). Being near the northern hemisphere winter solstice, it is most visible in northern hemisphere during summer.