Celestia Forums

Hi!

Sorry my bad English.
I have followed those forums for a long time, but this is my first post.
I have used Celestia to teach Astronomy here in Brazil.
The students get very astonished and it?s been very easy to teach them
using this worderful tool.
However there is two things that would help a lot if it was implemented:

1- Shadow cones
The same "engine" used to create comet tails would create a representation of the cone of shadow of planets and moons.
This feature would help a lot to explain why we don't have two eclipses
every month.
It woud be even better to have "umbra" and "penumbra" cones.

2- Time step keys
If we had keys to advance/return one day(solar and sidereal), we could show the shift of the stars relative to the Sun every day.
It would be nice to have key for years too.

Thank you very much and
Congratulations for you excelent work.

S?rgio Schmiegelow
Grupo de Estudos de Astronomia - UFSC
(Astronomy studies group of the Santa Catarina Federal University)

ssw wrote:2- Time step keys
If we had keys to advance/return one day(solar and sidereal), we could show the shift of the stars relative to the Sun every day.
It would be nice to have key for years too.

This should be fairly easy to handle by a script. I'll have a try...

Harald

Ok, see here:

http://www.shatters.net/forum/viewtopic.php?p=33651

But there are probably some errors, I am not sure how to work with the RotationPeriod and Period values provided in solarsys.ssc - anybody care to comment on how to correctly use these values to get normal daylength/sidereal daylength and year?

Hi Harry:

It works very fine!

Thank you very much!

S?rgio.

Harry wrote:I am not sure how to work with the RotationPeriod and Period values provided in solarsys.ssc - anybody care to comment on how to correctly use these values to get normal daylength/sidereal daylength and year?

RotationPeriod gives you the sidereal day in hours. Period gives you the sidereal year in terms of Earth's sidereal year, 365.25636 days.
To get the length of the solar day, you need some inverses:

1/SolarDay = 1/RotationPeriod - 1/(365.25636*24*Period)

which'll give you solar day in hours.

Grant

granthutchison wrote:RotationPeriod gives you the sidereal day in hours. Period gives you the sidereal year in terms of Earth's sidereal year, 365.25636 days.
To get the length of the solar day, you need some inverses:

1/SolarDay = 1/RotationPeriod - 1/(365.25636*24*Period)

which'll give you solar day in hours.

Thanks

But how long would an (earth-)year be, 365.25636 or 365.25 days (julian days=24h?), i.e. where is the 365.25636 coming from, I couldn't find it in the sources (only 365.25)?

It looks like 365.25636 is correct, at least position of earth is moving less with it when increasing time by a year (I assume the movement is due to the CustomOrbit, which may be the reason for the exact number)...

Harald

Harry wrote:But how long would an (earth-)year be, 365.25636 or 365.25 days (julian days=24h?), i.e. where is the 365.25636 coming from, I couldn't find it in the sources (only 365.25)?

It depends on what "year" you want.
365.25 days is the Julian year, which is just a calendrical convention ... it hasn't any relevance to the actual movement of the Earth around the Sun, except as an approximation that arises from our use of leap years.
365.25636 days is the sidereal year, which is the length of time the Earth takes to go once around the Sun relative to the fixed stars ... that's the time Celestia uses to measure an orbit.
But the Earth also precesses (which isn't yet implemented in Celestia). That means it's always twisting towards the Sun, so the seasons come on a little faster than they would in the absence of precession ... so we see the Sun return to the same position in the sky once every tropical year, or 365.2421897 days. It's the tropical year that most people would think of as being "a year", and all that fiddling with leap years and leap centuries is to keep the calendar in line with the tropical year.

Grant

Grant,

A confusion point for me:
what day length are you using for your values of the year?
Sidereal or solar?

selden wrote:Sidereal or solar?

Solar.

Grant

granthutchison wrote:365.25636 days is the sidereal year, which is the length of time the Earth takes to go once around the Sun relative to the fixed stars ... that's the time Celestia uses to measure an orbit.

That makes most sense in the context of the script, so I use it.

But the Earth also precesses (which isn't yet implemented in Celestia). That means it's always twisting towards the Sun, so the seasons come on a little faster than they would in the absence of precession ... so we see the Sun return to the same position in the sky once every tropical year, or 365.2421897 days. It's the tropical year that most people would think of as being "a year", and all that fiddling with leap years and leap centuries is to keep the calendar in line with the tropical year.

Ok, that's been confusing me. Thank you very much!

I will now upload the changed script...

Harald