Four Skies a Year
Because Earth orbits the Sun, the night faces a new part of the sky each season and every star rises about four minutes earlier each day, compounding to a completely turned-over sky across a year; the Sun's own yearly path through the stars is the ecliptic. · 13 min
Folio 2 left four minutes unspent. The sky returns to its exact starting position every 23 hours 56 minutes — four minutes short of your clock. That shortfall does nothing you would notice from one night to the next. Given a year, it rebuilds the entire evening sky: it is the reason Orion owns the winter nights and vanishes by summer. This folio spends the four minutes.
Guess before you learn
A particular star rises at exactly 10:00 tonight. Same star, same spot on your horizon — about when does it rise one month from tonight?
Four minutes a night sounds like nothing, and for one night it is. But it always runs the same direction — earlier — and it compounds: two hours a month, a full twenty-four hours across a year, which lands the star right back where it began. Most people expect a star to keep clock time. It keeps star time, and the two drift apart by exactly the four minutes folio 2 set aside.
9–12
3–5
Earth circles the Sun once a year, and at night you face away from the Sun into the dark half of the sky. As Earth moves along its orbit, that dark direction slowly swings around, so a different set of constellations fills each season's evening. Winter evenings hold Orion; six months later Orion is up in the daytime, lost in the Sun's glare, and summer brings other stars.
There is a matching daytime story. The Sun appears to drift across the same band of constellations over the year, spending about a month in each. That path is called the ecliptic — and later the Moon and planets will turn up along it too.
6–8
Earth orbits the Sun once a year, and at night you look into the half of the sky turned away from the Sun. As Earth rounds its orbit, that anti-Sun direction sweeps through the whole circle of stars, so each season's evening sky faces a different way. Orion rules winter evenings; by summer the Sun stands in front of that same patch, so Orion is a daytime object and Scorpius takes the opposite, summer sky.
The Sun's own yearly path against the background stars is the ecliptic — really the plane of Earth's orbit seen projected onto the sky. The Sun creeps eastward along it about 1 degree a day, through the constellations of the zodiac, and the Moon and planets are always found near that same line. Whichever constellation the Sun sits in is exactly the one you cannot see, hidden in the day.
9–12
Two motions combine. Earth's rotation turns the sky westward each night; Earth's orbit slides the whole nightly show about 1 degree — four minutes — earlier each day. The rotation you see within an hour; the orbital drift you notice across weeks. In a year the drift completes one full lap, which is why the star that rises at 10 p.m. tonight rises at 10 p.m. again next year on this date.
The ecliptic is the Sun's apparent yearly track through the stars, the projection of Earth's orbital plane. Because that plane also roughly contains the Moon's orbit and the planets' orbits, everything in the solar system parades along the ecliptic. Your midnight sky always faces the point directly opposite the Sun on it — so the midnight constellations are the ones the Sun will not reach for six months.
K–2
Earth travels a big circle around the Sun each year. At night you look away from the Sun, into the dark. As Earth moves along, the dark side faces new stars. So the night sky slowly changes.
Some stars belong to winter nights. Others belong to summer nights. Orion shines on cold winter evenings. By summer, Orion is hidden behind the Sun in the daytime, and new stars fill the night.
Undergrad
The seasonal sky is the orbital consequence of the sidereal-solar day difference. Because Earth advances about 0.986 degrees per day along its orbit, a given star returns to the meridian roughly 3 minutes 56 seconds earlier by the clock each day — the sidereal-day deficit. Integrated over a year that accumulated shift is exactly 24 hours, one full revolution, which restores the sky to its date-anchored appearance.
The ecliptic is the great circle where Earth's orbital plane meets the celestial sphere, inclined 23.4 degrees to the celestial equator; that tilt, not this folio's drift, is what makes seasons warm and cold. The Sun travels the ecliptic eastward at about 1 degree per day, and its right ascension climbing through 24 hours across the year is what carries each constellation out of the night sky and back again.
Postgrad
Quantitatively the mean solar day exceeds the mean sidereal day by the ratio 1 + 1/365.25, so stars transit 3 minutes 55.9 seconds earlier per solar day; the Sun's mean motion in right ascension is 360 degrees per 365.25 days. The apparent solar day itself breathes around the mean through the equation of time, the combined signature of orbital eccentricity and the 23.4-degree obliquity projecting ecliptic motion onto the equator.
The ecliptic is the instantaneous plane of the heliocentric orbit; the Moon's orbit is inclined about 5 degrees to it and the major planets within a few degrees, which is why the zodiac is a narrow belt and why eclipses and occultations happen at all. Precession of the equinoxes slides the Sun's zodiacal calendar backward one full sign every 2,150 years or so, so the tropical sign and the sidereal constellation no longer coincide.
ecliptic
The Sun's apparent path through the stars over a year — the plane of Earth's orbit projected on the sky. The Moon and planets are always found close to it.
The plate shows a whole year at once. Earth sits at four points on its orbit, and at each one your midnight sky faces straight out, away from the Sun. In winter that outward direction lands on Orion; roll halfway around the orbit to summer and the same direction now points at the Sun, so Orion is a daytime object, while midnight faces Scorpius instead. The season does not change the stars; it changes which way away from the Sun happens to point.
Watch one star across a year instead of one night. Pick a star that crosses your meridian — its highest point — at 9 o'clock this evening. Because it slips four minutes earlier each night, next month it crosses at 7, the month after at 5, and on. Before the ink draws the line, sketch the clock time of that meridian crossing for each of the next six months. Mind the direction of the drift.
When will a 10 p.m. star rise three months from now? — the steps fade as you master them
About 4 minutes.
4 min × 30 = 120 min = 2 hours earlier.
3 × 2 hours = 6 hours earlier.
4 p.m. three months from now — in daylight.
Why is this true?
Why does a star rise four minutes earlier each night?
Earth must spin an extra degree each day to face the Sun again after moving along its orbit, so the solar day runs about four minutes longer than one true turn against the stars — and the stars, keeping the shorter day, gain four minutes on the clock every night.
Unit I is complete. You can find north, read the sky's nightly turn, give any star a permanent address, and now say which stars each season brings. Everything so far has been fixed stars, wheeling in lockstep. The next unit turns to the things that break ranks — the Moon changing shape night by night, and the handful of bright wanderers that drift against the stars along the very ecliptic you just met.
Practice — new ink and old, interleaved
1.Without looking back: what is the difference between a star's altitude-azimuth and its right ascension-declination?
Altitude and azimuth say where a star is right now from where you stand and change every minute; right ascension and declination are its fixed address on the celestial sphere, the same for every observer and every date.
How close were you? Grade yourself honestly — it sets your review date.
2.The sky turns 15° per hour. Two stars 3 hours of right ascension apart cross your meridian how far apart in time?
3.The sky turns about 15° per hour. The four minutes a star gains each night corresponds to how many degrees?
4.A star crosses the meridian at 10 p.m. tonight. About when will it cross two months from now?
5.In one sentence: why do altitude and azimuth change through the night while right ascension and declination do not?
6.In one sentence: why is the constellation the Sun is currently in the one you cannot see?
7.Without looking back: what single motion explains the nightly movement of every star, and how fast does the sky appear to turn?
Earth's eastward spin — one rotation in 23 hours 56 minutes — makes the whole sky appear to wheel westward at about 15 degrees per hour.
How close were you? Grade yourself honestly — it sets your review date.
8.A star has declination +70°. From latitude 40°N, what does it do?
9.You want to tell a friend which star to find next season. Which coordinates stay valid?