Observing the March Equinox on the Astronomical Quadrant
2024-03-20
It’s equinox today, the day where the celestial equator and the ecliptic intersect. Today, Earth's tilt aligns with its orbit, and the Sun appears exactly above the planet's equator rather than toward the north or south.
While equinox is on Mar-20, the day with as close to 12 hours of daylight and a Sun hour as close to 60 minutes is on Mar-16. Around equinox, all Latitude Quadrants are accurate everywhere on earth.
Why the difference?
[Only] at the equator, the Sun is directly overhead at noon on these two equinoxes. The "nearly" equal hours of day and night are due to refraction of sunlight or a bending of the light's rays that causes the Sun to appear above the horizon when the actual position of the Sun is below the horizon. Additionally, the days become a little longer at the higher latitudes (those at a distance from the equator) because it takes the Sun longer to rise and set. Therefore, on the equinox and for several days before and after the equinox, the length of day will range from about 12 hours and six and one-half minutes at the equator, to 12 hours and 8 minutes at 30 degrees latitude, to 12 hours and 16 minutes at 60 degrees latitude.[1]
March equinox on a Wndsn Latitude Quadrant. Note that the (dashed) unequal hour lines (daytime hours / 12) are crossing (red circles) the equal hour lines (daytime + nighttime / 24) on that day, and only on that day.
For those of our readers with Astronomical (Latitude) Quadrants, you can take note of the equinox by:
- Aligning the string with the date of the equinox.
- When you do so, you find out that the degree arc shows the noon altitude of the Sun for this day, which equals
90° - your latitude. - Keeping the string in place, slide the cursor knot upwards to the 12-hour arc. This now marks the day on the string and you are witnessing a graphical calculation of the equinox. You can now delineate the sunrise and sunset for this date, thereby determining the amount of daylight available. Also by sighting the Sun with this set up, you can take the time.
Want to get more into the science and math of the equinox? Head over to the tutorials on Using the Obliquity Arc.
See also:
- The equinox on Tycho
- Observing the June Solstice
- GQT5 for in-depth discussions on the topic
References: