Archive

Posts Tagged ‘Tropic of Cancer’

Ephemeris: 03/12/2026 – Cancer the crab used to welcome summer

March 12, 2026 Comments off

This is Ephemeris for Thursday, March 12th. Today the Sun will be up for 11 hours and 44 minutes, setting at 7:45, and it will rise tomorrow at 7:58. The Moon, 1 day past last quarter, will rise at 5:25 tomorrow morning.

There’s a line on some globes and maps at approximately 23 1/2° north latitude called the Tropic of Cancer. It’s related to the constellation Cancer the crab. However, Cancer no longer fits that role that it was named to a couple of thousand years ago, when the sun entered the constellation of Cancer on the first day of summer. That’s the latitude on the Earth where the Sun was directly overhead on the first day of summer. Now that an honor goes to Gemini. The way we draw the figure of that constellation, the Sun is right near Castor’s big toe on the first day of summer. But I don’t think they’re going to change the name anytime soon. The reason for the change is that the Earth’s axis slowly wobbles like a top or gyroscope as they slow down. The effect is called precession.

The astronomical event times given in this blog are for the Traverse City/Interlochen area of Michigan (Lat 44.7° N, Long 85.7° W; EST, UT – 5 hours) unless stated otherwise. Times will be different for other locations.

Addendum

Comparison of the position of the Sun against the constellations on the summer solstice from 150 CE to now due to the precession of the equinoxes.
Comparison of the position of the Sun against the constellations on the summer solstice from 150 CE to now due to the precession of the equinoxes. The reason I chose 150 CE, is that it was the approximate date of Ptolemy’s Almagast, the standard work on astronomy until Copernicus in the 16th century or later. Created using Stellarium, LibreOffice Draw and GIMP.
How the tropics of Cancer and Capricorn line up with the Sun on the solstices.
How the tropics of Cancer and Capricorn line up with the Sun on the solstices. From an animation in Wikipedia, in the Public Domain.

The Greek letter epsilon in the image above represents the tilt of the Earth’s axis of around 23.5°. Astronomers call it the obliquity of the ecliptic, the angle between the ecliptic and the celestial equator as seen in the illustration below.

An animation of the precession of the equinoxes.
An animation of the precession of the equinoxes. The blue horizontal line is the celestial equator. The orange line is the ecliptic, the apparent annual path of the Sun against the stars. Where the two lines cross is the vernal equinox where the Sun is on the first day of spring, which on our calendar is trending to be March 20th. The slippage of the stars eastward (to the left) along the ecliptic is about the apparent width of the Sun or Moon, or half a degree, in 36 years. We’re looking at two different years 150 CE, the time of Ptolemy, and our time. To tell which is which, the one from our time has Saturn at the lower right. Jupiter happens to be in both of them, but it’s obviously been around lots of times between then and now. Created using Stellarium and GIMP.
Precesion animation
The 25,700-year cycle of precession as seen from near the Earth. The current North Pole star is Polaris (top). In about 8,000 years it will be the bright star Deneb (left), and in about 12,000 years, Vega (left center). The Earth’s rotation is not depicted to scale – in this span of time, it would actually rotate over 9 million times. Credit image Tfrooo, caption Wikipedia.
Categories: Concepts, Constellations, Ephemeris Program, Seasons, Uncategorized Tags:

06/20/2022 – Ephemeris – Here we are at the last full day of spring

June 20, 2022 Comments off

This is Ephemeris for Juneteenth, Observed, Monday, June 20th. Today the Sun will be up for 15 hours and 34 minutes, setting at 9:31, and it will rise tomorrow at 5:57. The Moon, at last quarter today, will rise at 2:09 tomorrow morning.

Here we are at the last full day of spring. Summer will begin at 5:14 EDT, or 9:14 UT tomorrow morning, when the Sun reaches its highest point on the celestial sphere, and directly over the northern latitude line called the Tropic of Cancer. At that time, folks at or north of the Arctic Circle at about 66 ½ degrees north latitude won’t see the Sun set. As it is, Interlochen is only about 4 degrees latitude south of the land of the all night twilight. It’s neat, around here in the western part of the Lower Michigan, to go out around midnight and see a bit of the last twilight glow near the north. Remember that around here, local or astronomical midnight occurs around 1:45 am. Ah politicians, aren’t they wonderful. And they’ve just made Daylight Saving Time permanent.

The astronomical event times given are for the Traverse City/Interlochen area of Michigan (EDT, UT – 4 hours). They may be different for your location.

Addendum

The event at 5:14 am EDT or 9:14 UT is called the summer solstice, or in deference to our Southern Hemisphere neighbors, the June solstice, because for them winter is starting. Solstice means “Sun stands still”. It doesn’t, of course. The sun is always moving eastward against the stars. However, if one checks the altitude of the Sun in the south at local noon each day, the Sun would move higher each day since the winter solstice until around June 21st, and go no further. It would slowly begin a retreat, day by day. That pause at the highest point is the solstice.

Categories: Ephemeris Program, Seasons, Time Tags: , ,