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JNow Vs J2000 Equinoxes (Answer From CloudyNights.com - Read On!)

From CN:

https://www.cloudynights.com/topic/630944-jnow-or-j2000/

JNow and J2000 are often referred to as epochs, but they are actually equinoxes.  "Epoch" refers to how the stars are related to each other based on their proper motion - independent of the coordinate system used - at a frozen point in time.  Equinox refers to the underlying coordinate system - which is changing due to multiple factors - the main one being the precession of the earth's axis - but also nutation and other things.

 

A key point is that you would never enter "JNow" in a book or a table - because it is changing fairly quickly each year - and not because the star or nebula is moving - but because of what the earth is doing.

 

The J2000 coordinates of Antares will be good for a very long time - because even if you assume Antares has no proper motion - i.e. motion with respect to other stars - you can convert its J2000 coordinates to JNow in the future because it is the same point in the sky - but two different coordinate systems.

 

Internally a mount will always need to know JNow if you want good pointing and good polar alignment.  But you can always look up J2000 coordinates in a book or table and have them converted to what the mount needs.  Some mounts want JNow, while others receive J2000 and do the conversion themselves.

 

For plate solving you need to include the effect of proper motion - so there will be a database of stars in some equinox - probably J2000 - and for a given image you need to apply proper motion to know the coordinates of the stars at the time of the image - and that is where epoch applies.  Then you can convert those coordinates, in the current epoch, from J2000 to JNow.  But epoch doesn't really apply to what a mount needs because proper motion is slow compared to the precession of the equinox - which is just a moving coordinate system.

 

J2000 and JNow are equinoxes - and refer to a coordinate system.  Epoch tells you how exactly the stars are laid out in the sky at a given time - as they move with respect to each other.

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2 comments

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    Keiron Smith

    JNOW and J2000 are two different time standards used in astronomy. JNOW stands for Julian Date (now), which is the current Julian Date based on the current time. J2000, on the other hand, is a fixed time standard defined as the Julian Date on January 1, 2000 at 12:00:00 Universal Time (UT1).
     
    The Julian Date (JD) is a time scale used in astronomy that counts the number of days that have elapsed since the initial epoch of 4713 BC. JNOW and J2000 are two different ways of expressing this time scale. JNOW is used to express the current time in JD, while J2000 is used as a reference point for calculations and to compare observations made at different times.
     
    In general, J2000 is often used in astronomy because it provides a consistent and well-defined reference point that can be used to compare observations made at different times. This is especially useful when analyzing data from long-term observations or when combining data from different sources.
     
    Overall, the main difference between JNOW and J2000 is that JNOW is a time-dependent reference point that changes every day, while J2000 is a fixed reference point that is used as a standard in astronomical calculations.

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    Dan Platt

    The vernal equinox (first point of Aries) forms the X axis of the ecliptic AND equatorial coordinate systems.  However, the Earth's axis precesses, and the line pointing to the vernal equinox along with it.

    The coordinates of stars generally don't change with respect to a fixed equinox point (e.g. epoch J2000 -- where it was pointing 2000/1/1).  Also, if you want to compute orbits, you need to refer all the observations to the same coordinate frame, such as epoch J2000.

    BUT if you are observing from the earth, and look at altitude-azimuth, you can translate that to right-ascension and declination -- related to the longitude and latitude coordinates you are observing from.  There is an angle between Greenwich 0 longitude and the vernal equinox, best approximated by the Greenwich apparent sidereal time (GAST).  If you add your longitude to that (west negative), it will take you to your local sidereal time.  You subtract the object's equatorial longitude (in hours) from your local sidereal time to get the hour angle of the object.  Equatorial mounts track the hour angle so that the scope's rotation cancels the Earth's rotation keeping the image at the same point AND unrotated.  The thing is, the equinox all that refers to is JNow!  Once you compute your coordinates in JNow, you have to rotate them to J2000 for any work involving different dates -- especially if they span multiple years.

    The problem with smart telescopes and smart software (perhaps "clever" is a better word) is that they're both trying to anticipate whether you're looking at JNow, or need J2000.

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