Hearing the sky from one place, facing east

Try it with two longitudes

Open the lab, leave the timestamp alone, and jump the place from London to Tokyo. The Sun’s azimuth will leap something like 135°. That is not a bug in the pan law. That is the Earth having rotated under the same UTC instant. Free accounts get five minutes a day to notice this. Members can sit with it.

Why east, not south

Most planispheres for the northern hemisphere put south at the bottom of the chart because that is how you hold a paper sky map over your head. Most stereo conventions put “front” in the center of the pan pot. Those two facts fight if you face south: east would be left, west right, and the rising point — the interesting edge of the sky — would sit on one ear only. Facing east puts the rising line in front of you. Bodies climb out of the left or right depending on whether they are north or south of east, which for the Sun is season and latitude, and for the Moon is a mess of inclination plus your own latitude, which is exactly the sort of mess worth hearing.

I am not claiming astronomers listen this way. They don’t. The east-facing listener is a product decision so that “place” has a consistent ear. If you are at 0°N, 0°E (the lab’s UTC default, a buoy in the Gulf of Guinea more than a city) the celestial equator still rises due east. At 34°S, Sydney, the same equator rises north of east and the winter Sun stays in the northern half of your sky, which means more left-ear Sun than a Californian gets. That is the latitude term doing work.

Azimuth, hour angle, the unromantic pipeline

For each body we take ecliptic longitude from the Kepler set (Moon from a short series), convert to right ascension and declination with mean obliquity 23.439279°, then to local hour angle with GMST + longitude. Altitude and azimuth are the usual spherical pair:

sin(alt) = sin φ sin δ + cos φ cos δ cos H
tan(az) uses the north-from-east convention with the sign on sin H flipped so 90° is east.

Pan = −cos(az). At az 0° (north) that is −1, full left. At 90° (east) it is 0. At 180° (south) +1. Front gain = 0.18 + 0.82 × (0.5 + 0.5 sin az), so east is 1.0 and west is 0.18. Elevation gain ramps from 0.10 below about −6° altitude to 1.0 around 46° up. Those numbers are not psychophysics. They are what kept moonrise from slapping the limiter and kept the Sun at midnight from pretending to be a floodlight. You can argue with the knees. The axes are the ones I will defend.

Tides are local too

Even if we never panned anything, a coordinate would still belong in this instrument. The equilibrium tide at a point depends on the zenith angle of the body. The lab stores a local pull a × (3 sin² alt − 1) / 2, the P2 factor. A body in the zenith stretches you one way; on the horizon, another. We do not currently drive the carrier amplitudes from that term alone — distance and syzygy still dominate — but the CRT altitude numbers are there so you can see when the geometry is “up” versus “set.” Contrast-with-now is the cheap version of a control: today’s sky under yesterday’s, same place.

UTC as a default place is a compromise. 0°, 0° is a real coordinate, not “nowhere.” If you never touch the lat/lon fields you are still standing in the water off Ghana, facing Mecca-ish, which is a strange picture and also consistent. Set your actual home. The tz for a custom longitude is a crude Etc/GMT offset, not IANA, so datetime-local near a political time zone boundary can be off by a DST hour. The sky math uses the geographic lon, not the tz name, so the pan stays honest even if the clock label is slightly rude.

Planets in the same field

Mercury through Neptune get the identical sky function. There is no special “astrology house” pan. If Mars is in the south-southwest for your place, it is a right-ear, slightly-behind tone, quiet because its tidal a is a rounding error until we raise it with the 0.18 gamma. Conjunction with the Sun (elongation small) still lives in the weight, so an inferior conjunction of Venus is not a new pitch, it is Venus sitting on the solar carrier’s doorstep, probably in the same direction as the Sun, which at noon is in front of you only if you are in the tropics and lucky. Most of the year the Sun at noon is south (northern hemisphere), so “in front” at local noon is a lie if you are facing east — the Sun is on your right. The lab will put it on the right. That mismatch between “the sun is in front of my workday” and “I am facing east” is the kind of small irritation that means the coordinate is doing its job.

Headphones, or you are wasting the map

Speakers sum left and right in the room. The azimuth law collapses to a slightly louder mid. Bluetooth that downmixes to mono is worse. This is the same constraint as every binaural page on the site; it is just more painful here because pan is half the instrument. Wired stereo, or a headset that actually keeps two channels. If one ear is weak, the sky will list.

We do not convolve a KEMAR HRTF. Interaural time difference is not modeled; only level and the binaural beat between Moon and Sun carriers. A body due east has no ITD in this engine. If you close your eyes you will not point at it with a compass. You will hear it as “not left, not right, and louder than west.” That is the fidelity budget.

How to read the CRT while you listen

SUN AZ and MOON AZ print a cardinal, a degree, and a tag: L, front, R, or behind. Planet pull bars are still ranked by tidal a, not by how stereo-interesting they are. Hover a name on the orrery and the hover line adds azimuth. If the Moon says ESE front and you are hearing it in the right-center, the map is working. If you are hearing it hard left, check that you did not leave a stale custom longitude in the fields — the place is sticky in localStorage under bbm_hear_place_v3.

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