8 replies to this topic

[zleonis]

Almanac sites (e.g. the US Naval Observatory) are all in agreement that the summer solstice this year happens on June 21 at 02:42 GMT

However, if I look at Sky Safari (6 Pro), the Sun appears to reach its maximum declination  of 23° 26' 14.4"  at ~3pm DST (~1900 GMT on 20 June).

 

What's going on here? Am I misunderstanding the definition of solstice? Is there some error in the databases or calculations used by Sky Safari? (this would surprise me, since I've always found it to be accurate for things like lunar occultations and phenomena involving Jovian moons).

 

Clearly not the most pressing query on cloudy nights, but I'd welcome any insights or speculation.

[btschumy]

Not sure I know the answer even though I was one of the authors of SkySafari.  I would have to bring Tim in to get a better answer.

 

I *think* the root of the problem may be the most sites are assuming you are at the center of Earth (geocentric location) when doing the calculation.  SkySafari always uses the topocentric location.  In this case the exact time depends on where you are on the Earth's surface.

 

You should also make sure you have your coordinate system set to JNow rather than J2000.  There will be slight differences between the two coordinate systems.

 

I was using the Mac SkySafari 6 to do the calculations.  In this case I could have the Info panel up while changing time by minutes or seconds.  I would find the time where the apparent motion of the Sun in Dec goes to 0.000"/day.

 

What is your location (latitude, longitude and elevation).  This can make a big difference.

 

Hope that helps.

 

ETA:

 

Oh I see you are in Richmond, VA.  If I use the SkySafari location for Richmond, I get the solstice happening at Jun 21 14:00:03 UTC (10 am tomorrow morning)

Edited by btschumy, 20 June 2025 - 04:53 PM.

[daw316]

not correct def of solstice.  use RA=6hr

[zleonis]

Not sure I know the answer even though I was one of the authors of SkySafari.  I would have to bring Tim in to get a better answer.

 

I *think* the root of the problem may be the most sites are assuming you are at the center of Earth (geocentric location) when doing the calculation.  SkySafari always uses the topocentric location.  In this case the exact time depends on where you are on the Earth's surface.

 

You should also make sure you have your coordinate system set to JNow rather than J2000.  There will be slight differences between the two coordinate systems.

 

I was using the Mac SkySafari 6 to do the calculations.  In this case I could have the Info panel up while changing time by minutes or seconds.  I would find the time where the apparent motion of the Sun in Dec goes to 0.000"/day.

 

What is your location (latitude, longitude and elevation).  This can make a big difference.

 

Hope that helps.

 

ETA:

 

Oh I see you are in Richmond, VA.  If I use the SkySafari location for Richmond, I get the solstice happening at Jun 21 14:00:03 UTC (10 am tomorrow morning)

Thanks for the detailed reply! Am I understanding correctly that the reason there are local variations in the time of the solstice (when the sun's apparent motion in declination reverses sign) is due to parallax? That hadn't occurred to me, but it makes perfect sense now. By the way, I love Sky Safari! Hard to think of a better way to spend $20 on amateur astronomy. 

[btschumy]

To be honest, I don’t know what the technical definition of “solstice” is.  Perhaps it is the farthest northern declination of the Sun when “viewed” from the center of the Earth.  Nevertheless, SkySafari is reporting the position of the Sun as viewed from your location.  Yes, that would be a parallax difference (and possibly interacting with the Earth’s rotation).

[Vic Menard]

https://neoprogrammi..._Calculator.php

 

Note that SkySafari and most almanacs use GMT (LST is 00h 00m 00s). I suspect your result was calculated using EDT (possibly the mean local time). SkySafari also shows the solar ecliptic longitude (90 degrees, 00m, 00s) at solstice. I got a 4 hour differential (EDT) from the GMT almanac time.

Edited by Vic Menard, 21 June 2025 - 05:25 PM.

[ButterFly]

Firstly, there definitely is parallax to worry about here.  The Sun's actual declination at any given point in time will depend on where one is looking from, whether that's the center of the Earth (as a hypothetical observer), or somewhere on its surface.  The coordinate system (JNow vs J2000.0) is another that will throw of the declination values.

 

But, an "equinox" or "solstice" occurs at the precise moment when the Sun's apparent ecliptic longitude is a multiple of 90 degrees.  That's from the Explanatory Supplement to the Astronomical Almanac, section 12.2.1.1 (3d Ed.).  Nearly everything reputable you find will be based on USNO's Astronomical Almanac, and the lovely book above is the "manual" for how that's made.

 

Using the apparent ecliptic longitude is the only reasonable way to precisely define the exact moment when these things occur.  On the equinoxes, for example, as defined above, the day length is actually longer that the night length for that date, due to the Sun's size, refraction, and observer parallax.  The actual date of equal day and night is a few days off of the actual equinox.  Using longest and shortest days won't throw of solstices like that, but it will only give you a date, rather than an exact moment in time.

[ButterFly]

So, using SS7 Pro, and fiddling with the time, I get an ecliptic longitude of around 90 degrees at the local time that corresponds to about Friday, June 21, at 0245 GMT.  That's pretty close to the 0242 GMT value.  There is no "apparent" in the listed ecliptic longitude, so it would only be a guess as to why its off by three whole minutes.  I won't be sending in any angry letters based on that, but if I did, I'm sure my letter would go right in a special filing cabinet:

 

[ButterFly]

not correct def of solstice.  use RA=6hr

Very close.  But, it's ecliptic longitude rather than Right Ascension.  Again, the issue is parallax.  The apparent RA will depend on where one is observing from, on top of the difference between the two different, relatively tilted, coordinate systems.