Ýôåìåðèäíîå âðåìÿ
Ýôåìåðè́äíîå âðǻìÿ, ET â ðàâíîìåðíàÿ øêàëà âðåìåíè, îñíîâàííàÿ íà îïðåäåëåíèè ñåêóíäû, ââåäåííîì â 1952 ãîäó íà 8 ñúåçäå Ìåæäóíàðîäíîãî Àñòðîíîìè÷åñêîãî Ñîþçà, êîòîðîå íå çàâèñèò îò èçìåíÿþùåéñÿ ñêîðîñòè âðàùåíèÿ Çåìëè.  1956 ãîäó Ãåíåðàëüíîé êîíôåðåíöèåé ïî ìåðàì è âåñàì (CGPM) ýòî îïðåäåëåíèå áûëî ðåêîìåíäîâàíî ê èñïîëüçîâàíèþ, à â 1960 ãîäó ýôåìåðèäíàÿ ñåêóíäà áûëà ïðèíÿòà çà áàçîâóþ åäèíèöó âðåìåíè â Ìåæäóíàðîäíîé Ñèñòåìå åäèíèö ÑÈ[1].  1967 ãîäó â ÑÈ áûëî ââåäåíî äðóãîå îïðåäåëåíèå ñåêóíäû, îñíîâàííîå íà àòîìíûõ ÷àñàõ.[2]  1984 ãîäó øêàëà ýôåìåðèäíîãî âðåìåíè ET â àñòðîíîìèè áûëà çàìåíåíà íà øêàëó TDT äèíàìè÷åñêîãî çåìíîãî âðåìåíè, êîòîðóþ â 2001 ãîäó ñìåíèëà øêàëà TT çåìíîãî âðåìåíè.
- Ìåõàíèçì â îáðàùåíèå Çåìëè â òå÷åíèå ãîäà âîêðóã Ñîëíöà.
- Ìàñøòàá â ïðîäîëæèòåëüíîñòü îäíîé ýôåìåðèäíîé ñåêóíäû, ðàâíîé 1/31556925.9747 òðîïè÷åñêîãî ãîäà. Òàê êàê òðîïè÷åñêèé ãîä íå ÿâëÿåòñÿ ïîñòîÿííûì, òî çà ýòàëîí ïðèíÿòà ïðîäîëæèòåëüíîñòü òðîïè÷åñêîãî ãîäà â ôóíäàìåíòàëüíóþ ýïîõó 1900.0 ÿíâ.0, 12h ÅÒ., â òàáëèöàõ Íüþêîìáà[3]
- Íóëüïóíêò â ôóíäàìåíòàëüíàÿ ýïîõà 0 ÿíâàðÿ 1900[4], 12h ÅÒ íà íà÷àëüíîì ìåðèäèàíå.
- Ñïîñîá îòñ÷åòà â ÷åðåç ïîñðåäñòâî ñèñòåìû Âñåìèðíîãî âðåìåíè UT, ïðèáàâëåíèåì ïîïðàâêè íà ïåðåõîä ê ýôåìåðèäíîìó âðåìåíè:
ET = UT + ΔT,
ãäå ΔT (Äåëüòà T) â ïîïðàâêà íà âåêîâîå çàìåäëåíèå âðàùåíèÿ Çåìëè, êîòîðóþ â íàøå âðåìÿ îïðåäåëÿþò èçìåðåíèåì ïîëîæåíèÿ Çåìëè îòíîñèòåëüíî âíåãàëàêòè÷åñêèõ ðàäèîèñòî÷íèêîâ.
 ïåðâîì ïðèáëèæåíèè, ñèñòåìó ÅÒ ìîæíî ïðåäñòàâëÿòü êàê ñèñòåìó, îñíîâàííóþ íà ñóòî÷íîì âðàùåíèè Çåìëè, íî èñïðàâëåííóþ èç-çà íåðàâíîìåðíîñòè ýòîãî âðàùåíèÿ.
Òàê êàê ýôåìåðèäíàÿ ñåêóíäà ïðèâÿçàíà ê ïðîäîëæèòåëüíîñòè âïîëíå îïðåäåëåííîãî ãîäà, ýòàëîí ÅÒ íå ìîæåò áûòü âîñïðîèçâåäåí â ýòî èäåàëüíîå ïîñòðîåíèå.
Ñîäåðæàíèå |
[ïðàâèòü] Èñòîðèÿ ýôåìåðèäíîãî âðåìåíè
Ýôåìåðèäíîå âðåìÿ (ET), ââåä¸ííîå êàê ñòàíäàðò â 1952 ãîäó, èñõîäíî ðàçðàáàòûâàëîñü êàê ïðèáëèæåíèå ê îäíîðîäíîé âðåìåííîé øêàëå, îñâîáîæä¸ííîé îò ýôôåêòîâ íåðåãóëÿðíîñòè âðàùåíèÿ Çåìëè, «äëÿ óäîáñòâà àñòðîíîìîâ è äðóãèõ ó÷¸íûõ», íàïðèìåð, äëÿ èñïîëüçîâàíèÿ â ýôåìåðèäàõ Ñîëíöà, Ëóíû è ïëàíåò. Îíî áûëî ïðåäëîæåíî â 1948 ãîäó Äæ.Ì. Êëåìåíñîì[5].
Ñî âðåì¸í Äæîíà Ôëåìñòèäà (1646â1719) ñ÷èòàëîñü, ÷òî äíåâíîå âðàùåíèå Çåìëè ðàâíîìåðíî. Íî â êîíöå XIX â íà÷àëå XX âåêà ñ óâåëè÷åíèåì òî÷íîñòè àñòðîíîìè÷åñêèõ íàáëþäåíèé ñíà÷àëà ïðèøëî ïîäîçðåíèå, à çàòåì è áûëî äîêàçàíî, ÷òî âðàùåíèå Çåìëè (òî åñòü äëèíà äíÿ) èñïûòûâàåò êîëåáàíèÿ íà êîðîòêèõ âðåìåííûõ èíòåðâàëàõ, à â öåëîì çàìåäëÿåòñÿ. Äîêàçàòåëüñòâà ýòîãî ôàêòà áûëè ñóììèðîâàíû äå Ñèòòåðîì â 1927 ãîäó:
Åñëè ìû ïðèìåì ýòó ãèïîòåçó, òî «àñòðîíîìè÷åñêîå âðåìÿ», çàäàâàåìîå âðàùåíèåì Çåìëè è èñïîëüçóåìîå âî âñåõ ïðàêòè÷åñêèõ àñòðîíîìè÷åñêèõ âû÷èñëåíèÿõ, îòëè÷àåòñÿ îò «ðàâíîìåðíîãî» èëè «íüþòîíîâîãî» âðåìåíè, êîòîðîå îïðåäåëåíî êàê íåçàâèñèìàÿ ïåðåìåííàÿ óðàâíåíèé íåáåñíîé ìåõàíèêè.Îðèãèíàëüíûé òåêñò (àíãë.)If we accept this hypothesis, then the â˜astronomical timeâ™, given by the earthâ™s rotation, and used in all practical astronomical computations, differs from the â˜uniformâ™ or â˜Newtonianâ™ time, which is defined as the independent variable of the equations of celestial mechanics.
â [6]
Äå Ñèòòåð ïðåäëîæèë äëÿ ïîëó÷åíèÿ ðàâíîìåðíîãî âðåìåíè ïðèìåíÿòü ïîïðàâêó ê ñðåäíåìó ñîëíå÷íîìó âðåìåíè, äàâàåìîìó âðàùåíèåì Çåìëè.
Èñïîëüçóÿ ðåçóëüòàòû Ã.Ñ. Äæîíñà (1939)[7] Äæ. Ì. Êëåìåíñ â 1948 ãîäó [5] ïðåäëîæèë äåòàëüíóþ ñõåìó ïîñòîðîåíèÿ ðàâíîìåðíîé øêàëû.  ýòîé ðàáîòå Êëåìåíñ ïîÿñíèë, ÷òî òàêàÿ øêàëà ïðåäíàçíà÷åíà «òîëüêî äëÿ óäîáñòâà àñòðîíîìîâ è äðóãèõ ó÷¸íûõ» è ÷òî «äëÿ ãðàæäàíñêèõ íóæä ëîãè÷íî ïðîäîëæàòü èñïîëüçîâàòü ñðåäíåå ñîëíå÷íîå âðåìÿ»[8].
Äå Ñèòòåð è Êëåìåíñ íàçûâàëè ïðåäëîæåííîå âðåìÿ «íüþòîíîâûì» èëè «ðàâíîìåðíûì». Ä. Áðàóýð ïðåäëîæèë íàçâàíèå «ýôåìåðèäíîå âðåìÿ» [9].
Ñëåäóÿ ýòèì ïðåäëîæåíèÿì, àñòðîíîìè÷åñêàÿ êîíôåðåíöèÿ â Ïàðèæå (1950 ãîä) ðåêîìåíäîâàëà, «÷òî âî âñåõ ñëó÷àÿõ, êîãäà ñðåäíÿÿ ñîëíå÷íàÿ ñåêóíäà íåóäîâëåòâîðèòåëüíà êàê åäèíèöà âðåìåíè èç-çà å¸ âàðèàáåëüíîñòè, ïðèìåíÿåìîé åäèíèöåé [âðåìåíè] äîëæåí áûòü ñèäåðè÷åñêèé ãîä 1900.0, [è] ÷òî âðåìÿ, îòñ÷èòûâàåìîå â ýòèõ åäèíèöàõ, áóäåò îáîçíà÷àòüñÿ êàê âýôåìåðèäíîå âðåìÿâ»[10], è óòâåðäèëà ôîðìóëó Êëåìåíñà (ñì. Îïðåäåëåíèå ýôåìåðèäíîãî âðåìåíè (1952)) äëÿ ïåðåõîäà îò ñðåäíåãî ñîëíå÷íîãî âðåìåíè ê ýôåìåðèäíîìó.
Ìåæäóíàðîäíûé àñòðîíîìè÷åñêèé ñîþç ïîäòâåðäèë ýòó ðåêîìåíäàöèþ íà VIII Ãåíåðàëüíîé àññàìáëåå â 1952 ãîäó[9][11]. Ïðàêòè÷åñêîå ââåäåíèå øêàëû çàíÿëî íåêîòîðîå âðåìÿ (ñì. Èñïîëüçîâàíèå ýôåìåðèäíîãî âðåìåíè â îôèöèàëüíûõ àëüìàíàõàõ è ýôåìåðèäàõ), à çàòåì ýôåìåðèäíîå âðåìÿ ET îñòàâàëîñü ñòàíäàðòîì, ïîêà â 1970-õ íå áûëî çàìåíåíî áîëåå òî÷íûìè âðåìåííûìè øêàëàìè (ñì. Ïåðåñìîòð).
Âî âðåìÿ èñïîëüçîâàíèÿ ET êàê ñòàíäàðòà ýôåìåðèäíîå âðåìÿ ïîäâåðãàëîñü íåçíà÷èòåëüíûì èçìåíåíèÿì. Åäèíèöà âðåìåíè áûëà ïåðåîïðåäåëåíà ÷åðåç òðîïè÷åñêèé ãîä 1900.0 âìåñòî ñèäåðè÷åñêîãî[9], à ñòàíäàðòíàÿ ñåêóíäà ñíà÷àëà îïðåäåëÿëàñü êàê 1/31556925,975 òðîïè÷åñêîãî ãîäà 1900.0[9][12], à çàòåì êàê 1/31556925,9747[13], ïîêà â 1967/8 îíà íå áûëà ïåðåîïðåäåëåíà ÷åðåç öåçèåâûé ñòàíäàðò ÷àñòîòû (ñì. íèæå).
Õîòÿ ET ñåé÷àñ íàïðÿìóþ íå ïðèìåíÿåòñÿ, ýòà øêàëà îñòàâèëà çàìåòíûé ñëåä â íàóêå. Ÿ ïðååìíèêè, òàêèå êàê TDT è àòîìíàÿ øêàëà âðåìåíè TAI, áûëè ðàçðàáîòàíû òàê, ÷òîáû îáåñïå÷èòü íåïðåðûâíîñòü ñ ýôåìåðèäíûì âðåìåíåì («provides continuity with ephemeris time»)[14]. ET èñïîëüçîâàëîñü äëÿ êàëèáðîâêè àòîìíûõ ÷àñîâ â 1950-õ ãã.[15] Ñîîòâåòñòâèå ýôåìåðèäíîé ñåêóíäû ñ ñåêóíäîé ÑÈ, îïðåäåë¸ííîé ïîñðåäñòâîì öåçèåâûõ àòîìíûõ ÷àñîâ, áûëî ïðîâåðåíî ñ òî÷íîñòüþ âûøå 1 ê 1010[16].
Òàêèì îáðàçîì, ðåøåíèÿ ðàçðàáîò÷èêîâ øêàëû ýôåìåðèäíîãî âðåìåíè îïðåäåëèëè äëèíó íûíåøíåé ñåêóíäû, ÷òî, â ñâîþ î÷åðåäü, âëèÿåò íà êîëè÷åñòâî ñåêóíä êîîðäèíàöèè, êîòîðûå âñòàâëÿþòñÿ â ñîâðåìåííûå ãðàæäàíñêèå âðåìåííûå øêàëû äëÿ ïîääåðæàíèÿ èõ áëèçîñòè ê ñðåäíåìó ñîëíå÷íîìó âðåìåíè.
[ïðàâèòü] Îïðåäåëåíèå ýôåìåðèäíîãî âðåìåíè (1952)
Ýôåìåðèäíîå âðåìÿ áûëî â ïðèíöèïå îïðåäåëåíî ÷åðåç îðáèòàëüíîå äâèæåíèå Çåìëè âîêðóã Ñîëíöà[9], íî åãî ïðàêòè÷åñêàÿ ðåàëèçàöèÿ îáû÷íî ïîëó÷àëàñü èíûì îáðàçîì.
Äåòàëüíîå îïðåäåëåíèå ET îñíîâûâàëîñü íà Ñîëíå÷íûõ òàáëèöàõ (àíãë.)ðóññê. Ñ. Íüþêîìáà (1895)[3], èíòåðïðåòèðîâàííûõ íîâûì ñïîñîáîì äëÿ ó÷¸òà íåêîòîðûõ íàéäåííûõ îòêëîíåíèé îò íèõ:
Âî ââåäåíèè ê Òàáëèöàì (p. 9) áàçèñíûå ôîðìóëû âêëþ÷àëè âåëè÷èíó ñðåäíåé äîëãîòû Ñîëíöà âî âðåìÿ, îïðåäåëÿåìîå èíòåðâàëîì T, èçìåðÿåìûì â Þëèàíñêèõ ñòîëåòèÿõ èç 36525 ñðåäíèõ ñîëíå÷íûõ ñóòîê[17], îòñ÷èòàííûõ îò Ñðåäíåãî Ãðèíâè÷ñêîãî Ïîëóäíÿ (Greenwich Mean Noon) 0-ãî ÿíâàðÿ 1900 ãîäà:
. . . . . (1)
 1939 ãîäó Ñïåíñåð Äæîíñ[7] ïîêàçàë, ÷òî äåéñòâèòåëüíûå íàáëþäàåìûå ïîëîæåíèÿ Ñîëíöà ïðè ñðàâíåíèè ñ ïîëó÷àåìûìè èç ôîðìóëû Íüþêîìáà ïîêàçûâàþò ñèñòåìàòè÷åñêèå ðàñõîæäåíèÿ, âûðàæàåìûå ôîðìóëîé:
ãäå «âðåìÿ íàáëþäåíèÿ åñòü Âñåìèðíîå âðåìÿ UT, íå ñêîððåêòèðîâàííîå ê íüþòîíîâó âðåìåíè», à
ïðåäñòàâëÿåò ñîáîé íåðåãóëÿðíóþ ôëóêòóàöèþ, îïðåäåëÿåìóþ èç íàáëþäåíèé Ëóíû[18].
Òàêèì îáðàçîì óñëîâíî ïðàâèëüíàÿ ôîðìóëà äîëæíà áûëà áûòü ñóììîé ýòèõ âûðàæåíèé:
. . . . . (2)
Ïðåäëîæåíèå Êëåìåíñà (1948) ñîñòîÿëî íå â ïðèìåíåíèè êîððåêöèè ïîäîáíîãî òèïà èñõîäÿ èç ñðåäíåãî ñîëíå÷íîãî âðåìåíè, à íàîáîðîò â â îïðåäåëåíèè íîâîé âðåìåííîé øêàëû E, èñõîäÿ èç îðèãèíàëüíîé ôîðìóëû Íüþêîìáà (1) è ðåàëüíîãî ïîëîæåíèÿ Ñîëíöà:
. . . . . (3)
ãäå ñåé÷àñ âðåìåííàÿ ïåðåìåííàÿ E ïðåäñòàâëÿåò ñîáîé âðåìÿ, îòñ÷èòàííîå â ýôåìåðèäíûõ ñòîëåòèÿõ (èç 36525 ýôåìåðèäíûõ äíåé èç 86400 ýôåìåðèäíûõ ñåêóíä) îò Ñðåäíåãî Ãðèíâè÷ñêîãî Ïîëóäíÿ (Greenwich Mean Noon) 0-ãî ÿíâàðÿ 1900 ãîäà. Îôèöèàëüíîå îïðåäåëåíèå 1961 ãîäà ãëàñèò: «Íà÷àëî îòñ÷¸òà è ñêîðîñòü òå÷åíèÿ ýôåìåðèäíîãî âðåìåíè îïðåäåëåíû òàê, ÷òîáû ñðåäíÿÿ äîëãîòà Ñîëíöà ñîâïàäàëà ñ âûðàæåíèåì Íüþêîìáà»[19].
Èç ñðàâíåíèÿ ôîðìóë (2) è (3), âûðàæàþùèõ îäíî è òî æå äâèæåíèå Ñîëíöà, íî îòñ÷èòàííîå â ðàçíûõ âðåìåííûõ øêàëàõ, Êëåìåíñ ïîëó÷èë íåÿâíîå âûðàæåíèå, îöåíèâàþùåå ðàçíèöó â ñåêóíäàõ ìåæäó ýôåìåðèäíûì âðåìåíåì è ñðåäíèì ñîëíå÷íûì âðåìåíåì
[18]:
. . . . . (4)
Ôîðìóëà Êëåìåíñà, ñåé÷àñ çàìåí¸ííàÿ áîëåå òî÷íûìè îöåíêàìè, áûëà âêëþ÷åíà â èñõîäíîå ðåøåíèå êîíôåðåíöèè ïî ýôåìåðèäíîìó âðåìåíè. Èç-çà ôëóêòóàöèîííîãî ÷ëåíà ïðàêòè÷åñêîå îïðåäåëåíèå ðàçíèöû ìåæäó ET è UT çàâèñåëî îò íàáëþäåíèé. Ñðàâíåíèå ôîðìóë âûøå ïîêàçûâàåò, ÷òî (èäåàëüíî ïîñòîÿííàÿ) åäèíèöà ýôåìåðèäíîãî âðåìåíè, íàïðèìåð, ýôåìåðèäíàÿ ñåêóíäà, âåñü XX âåê áûëà ÷óòü-÷óòü êîðî÷å ñîîòâåòñòâóþùåé (íî íå èäåàëüíî ïîñòîÿííîé) åäèíèöû ñðåäíåãî ñîëíå÷íîãî âðåìåíè (êîòîðàÿ ïîìèìî íåðåãóëÿðíûõ ôëóêòóàöèé èñïûòûâàåò ïîñòåïåííîå ñòàáèëüíîå óâåëè÷åíèå), â ñîîòâåòñòâèè òàêæå ñ áîëåå íîâûìè ðåçóëüòàòàìè Ìîððèñîíà è Ñòåôåíñîíà[20] (ñì. ΔT).
[ïðàâèòü] Ðåàëèçàöèè
[ïðàâèòü] Âòîðè÷íàÿ ðåàëèçàöèÿ íà îñíîâå íàáëþäåíèé Ëóíû
Õîòÿ ýôåìåðèäíîå âðåìÿ â ïðèíöèïå îïðåäåëåíî ÷åðåç îðáèòàëüíîå äâèæåíèå Çåìëè âîêðóã Ñîëíöà[21], íà ïðàêòèêå îíî îáû÷íî èçìåðÿëîñü ïî îðáèòàëüíîìó äâèæåíèþ Ëóíû âîêðóã Çåìëè[22]. Ýòè èçìåðåíèÿ ïîñëå êàëèáðîâêè ñðåäíåãî äâèæåíèÿ Ëóíû ïî îòíîøåíèþ ê ñðåäíåìó äâèæåíèþ Ñîëíöà ìîæíî ðàññìàòðèâàòü êàê âòîðè÷íûé ñòàíäàðò (â ìåòðîëîãè÷åñêîì ñìûñëå) øêàëû ET[23].
Ïðè÷èíîé èñïîëüçîâàíèÿ äâèæåíèÿ Ëóíû áûëî ïðàêòè÷åñêîå óäîáñòâî: òàê êàê ñêîðîñòü äâèæåíèÿ Ëóíû íà ôîíå çâ¸çä ïðèìåðíî â 13 ðàç âûøå ñêîðîñòè äâèæåíèÿ Ñîëíöà, ïðè òîé æå òî÷íîñòè èçìåðåíèÿ ïîëîæåíèÿ Ëóíû ýôåìåðèäíîå âðåìÿ îïðåäåëÿëîñü âî ñòîëüêî æå ðàç òî÷íåå.
Êîãäà ýôåìåðèäíîå âðåìÿ òîëüêî ââîäèëîñü, âðåìåííûå øêàëû âñ¸ åù¸ áàçèðîâàëèñü íà àñòðîíîìè÷åñêèõ íàáëþäåíèÿõ. Òî÷íîñòü øêàë áûëà îãðàíè÷åíà òî÷íîñòüþ îïòè÷åñêèõ íàáëþäåíèé, à ïîïðàâêè êî âðåìåíè îïðåäåëÿëèñü ïîñòôàêòóì.
[ïðàâèòü] Âòîðè÷íàÿ ðåàëèçàöèÿ íà îñíîâå àòîìíûõ ÷àñîâ
×åðåç íåñêîëüêî ëåò ïîñëå èçîáðåòåíèÿ öåçèåâûõ àòîìíûõ ÷àñîâ ïîÿâèëàñü àëüòåðíàòèâà àñòðîíîìè÷åñêîé ðåàëèçàöèè øêàëû ýôåìåðèäíîãî âðåìåíè. Ïîñëå êàëèáðîâêè øêàëû àòîìíîãî âðåìåíè ïî ET (1958 ãîä[15]) öåçèåâûå ÷àñû ñòàëè èñïîëüçîâàòüñÿ äëÿ õðàíåíèÿ ýôåìåðèäíîãî âðåìåíè. Àòîìíûå ÷àñû ïðåäîñòàâèëè íîâóþ âòîðè÷íóþ ðåàëèçàöèþ ET ïî÷òè â ðåàëüíîì âðåìåíè[23], ÷òî âñêîðå äîêàçàëî ñâîþ ïîëåçíîñòü è ïðåèìóùåñòâà îòíîñèòåëüíî ïåðâè÷íîãî ñòàíäàðòà ET: íå òîëüêî áîëüøåå óäîáñòâî, íî è ëó÷øóþ ðàâíîìåðíîñòü õîäà âðåìåíè îòíîñèòåëüíî ýôåìåðèäíîãî. Òàêèå ðåàëèçàöèè òàêæå èñïîëüçîâàëèñü è íàçûâàëèñü êàê 'ET', õîòÿ è ïîäðóçóìåâàëîñü, ÷òî àòîìíûå âðåìåííûå øêàëû íå ïîëíîñòüþ èäåíòè÷íû ïåðâè÷íîìó ñòàíäàðòó ýôåìåðèäíîãî âðåìåíè, íî ñêîðåå ïðåäñòàâëÿþò ñîáîé åãî óëó÷øåííûå âåðñèè â ñìûñëå áîëüøåé ðàâíîìåðíîñòè õîäà[24]. Àòîìíûå ÷àñû çàòåì äàëè íà÷àëî øêàëå àòîìíîãî âðåìåíè, è øêàëå, êîòîðàÿ ñíà÷àëà íàçûâàëàñü «Çåìíîå äèíàìè÷åñêîå âðåìÿ», à ñåé÷àñ «Çåìíîå âðåìÿ», îïðåäåë¸ííîé òàê, ÷òîáû îáåñïå÷èòü íåïðåðûâíîñòü ñî øêàëîé ET[14].
Äîñòóïíîñòü àòîìíûõ ÷àñîâ âìåñòå ñ óâåëè÷èâàþùåéñÿ òî÷íîñòüþ àñòðîíîìè÷åñêèõ íàáëþäåíèé (êîòîðàÿ ïðèøëà ê òîìó ìîìåíòó, êîãäà ðåëÿòèâèñòñêèå ïîïðàâêè óæå íå ïðåíåáðåæèìî ìàëû)[25], ïðèâåëà ê çàìåùåíèþ ñòàíäàðòà ýôåìåðèäíîãî âðåìåíè áîëåå òî÷íûìè øêàëàìè, âêëþ÷àÿ Çåìíîå âðåìÿ è Áàðèöåíòðè÷åñêîå äèíàìè÷åñêîå âðåìÿ, ê êîòîðûì ñàìî ET ìîæåò ðàññìàòðèâàòüñÿ êàê ìåíåå òî÷íîå ïðèáëèæåíèå.
[ïðàâèòü] Ïåðåîïðåäåëåíèå âðåìåííûõ øêàë
 ðåçîëþöèè 1976 ãîäà Ìåæäóíàðîäíûé àñòðîíîìè÷åñêèé ñîþç îòìåòèë, ÷òî òåîðåòè÷åñêèé áàçèñ òåêóùåé ðåàëèçàöèè (1952) ñòàíäàðòà ýôåìåðèäíîãî âðåìåíè ÿâëÿëñÿ íåðåëÿòèâèñòñêèì, è ïîýòîìó, íà÷èíàÿ ñ 1984 ãîäà, ýôåìåðèäíîå âðåìÿ áóäåò çàìåíåíî äâóìÿ ðåëÿòèâèñòñêèìè øêàëàìè äèíàìè÷åñêîãî âðåìåíè: Çåìíûì äèíàìè÷åñêèì âðåìåíåì TDT è Áàðèöåíòðè÷åñêèì äèíàìè÷åñêèì âðåìåíåì TDB[26]. Âîçíèêøèå òðóäíîñòè áûëè ðàçðåøåíû ïåðåõîäîì â 1990-õ ãã. ê íîâûì øêàëàì Çåìíîãî âðåìåíè TT, Geocentric Coordinate Time GCT (TCG) è Barycentric Coordinate Time BCT (TCB)[14].
[ïðàâèòü] Èñïîëüçîâàíèå ýôåìåðèäíîãî âðåìåíè â îôèöèàëüíûõ àëüìàíàõàõ è ýôåìåðèäàõ
Ýôåìåðèäíîå âðåìÿ ïî ñòàíäàðòó 1952 ãîäà èñïîëüçîâàëîñü â «Astronomical Ephemeris» (UK) è â American Ephemeris and Nautical Almanac, çàìåíèâ UT â îñíîâíûõ ýôåìåðèäàõ, íà÷èíàÿ ñ 1960 ãîäà[27]. (Îäíàêî ýôåìåðèäû â «Nautical Almanac», ñ òåõ ïîð â îòäåëüíîì èçäàíèè äëÿ øòóðìàíîâ, ïðîäîëæàþò âûðàæàòüñÿ â UT.) Ýôåìåðèäû ïóáëèêîâàëèñü â ET âïëîòü äî 1983 ãîäà âêëþ÷èòåëüíî (ñ íåêîòîðûìè èçìåíåíèÿìè â ñâÿçè ñ èçìåíåíèÿìè àñòðîíîìè÷åñêèõ ïîñòîÿííûõ), ïîñëå ÷åãî íà÷èíàÿ ñ 1984 ãîäà, èñïîëüçóþòñÿ ýôåìåðèäû JPL.
Åù¸ äî ïåðåõîäà â 1960 ãîäó íà ET â àëüìàíàõàõ «Improved Lunar Ephemeris» áûëè ïåðåñ÷èòàíû íà ET äëÿ ïåðèîäà 1952â1959 ãã.[28] Øàáëîí:Ýêåðò, Óîëëåñ Äæîí èñõîäÿ èç òåîðèè äâèæåíèÿ Ëóíû Áðàóíà ñ ìîäèôèêàöèÿìè, ðåêîìåíäîâàííûìè Êëåìåíñîì (1948).
[ïðàâèòü] Ïåðåîïðåäåëåíèå ñåêóíäû
Ïîñëåäîâàòåëüíûå ïåðåîïðåäåëåíèÿ åäèíèöû ýôåìåðèäíîãî âðåìåíè îïèñàíû â ðàçäåëå «Èñòîðèÿ». Âåëè÷èíà ñòàíäàðòíîé ñåêóíäû 1956/1960 ãîäà:
- 1/31 556 925,9747 òðîïè÷åñêîãî ãîäà äëÿ 0 ÿíâàðÿ 1900 ãîäà â 12 ÷àñîâ ýôåìåðèäíîãî âðåìåíè,
áûëî ïîëó÷åíî èç ëèíåéíîé ïî âðåìåíè ÷àñòè ôîðìóëû Íüþêîìáà (1) (âçÿòîé â ñìûñëå ôîðìóëû (3)). Ñîîòíîøåíèå ñ êîýôôèöèåíòîì Íüþêîìáà âèäíî èç âûðàæåíèÿ:
- 1/31 556 925,9747 = 129 602 768,13 / (360×60×60×36525×86400).
Ìîíèòîðèíã öåçèåâûõ àòîìíûõ ÷àñîâ ñ 1955 ãîäà áûñòðî ïîêàçàë, ÷òî ñêîðîñòü âðàùåíèÿ Çåìëè èñïûòûâàåò ôëóêòóàöèè. Ýòî ïîäòâåðäèëî íåïðèåìëåìîñòü ñðåäíåé ñîëíå÷íîé ñåêóíäû Âñåìèðíîãî âðåìåíè UT êàê åäèíèöû èçìåðåíèÿ äëÿ íàèáîëåå òî÷íûõ èçìåðåíèé. Ïîñëå òð¸õ ëåò ñðàâíåíèÿ øêàëû àòîìíîãî âðåìåíè ñ ëóííûìè íàáëþäåíèÿìè Â. Ìàðêîâèòö (àíãë.)ðóññê. ñ ñîàâò. (1958) îïðåäåëèëè, ÷òî ýôåìåðèäíàÿ ñåêóíäà ñîîòâåòñòâóåò 9 192 631 770 ± 20 ïåðèîäàì êîëåáàíèé èçëó÷åíèÿ äëÿ âûáðàííîãî â êà÷åñòâå ÷àñîâîãî ïåðåõîäà â àòîìå öåçèÿ[15].
Ñëåäóÿ ýòèì èçìåðåíèÿì, â 1967/68 ãã. Ãåíåðàëüíàÿ êîíôåðåíöèÿ ïî ìåðàì è âåñàì çàìåíèëà îïðåäåëåíèå ñåêóíäû ÑÈ íà ñëåäóþùåå:
Ñåêóíäà ðàâíà 9 192 631 770 ïåðèîäàì èçëó÷åíèÿ, ñîîòâåòñòâóþùåãî ïåðåõîäó ìåæäó äâóìÿ ñâåðõòîíêèìè óðîâíÿìè îñíîâíîãî ñîñòîÿíèÿ àòîìà öåçèÿ-133.
Õîòÿ ýòî îïðåäåëåíèå ïîëíîñòüþ íåçàâèñèìî îò îïðåäåëåíèÿ ýôåìåðèäíîé ñåêóíäû, îíî îïðåäåëÿåò ñåêóíäó, íåîòëè÷èìóþ ïî âåëè÷èíå îò ýôåìåðèäíîé (êàê ýòî èçìåðåíî â 1958 ãîäó àòîìíûìè ÷àñàìè). Ïîçäíåå, â 1988 ãîäó, áûëî ïðîâåäåíî åù¸ îäíî ñëè÷åíèå ñåêóíä (Markowitz, 1988[16]) è îêàçàëîñü, ÷òî ðàçíîñòü àòîìíîé öåçèåâîé è ýôåìåðèäíîé ñåêóíäû (îïðåäåë¸ííîé èç íàáëþäåíèé Ëóíû) íå ïðåâûøàåò 1 ê 1010.
Äëÿ ïðàêòè÷åñêèõ ïðèìåíåíèé äëèíà ýôåìåðèäíîé ñåêóíäû ìîæåò áûòü ïðèíÿòà ðàâíîé äëèòåëüíîñòè ñåêóíäû Áàðèöåíòðè÷åñêîãî äèíàìè÷åñêîãî âðåìåíè (TDB), Çåìíîãî âðåìåíè (TT) èëè ïðåäøåñòâóþùåãî TT â Çåìíîãî äèíàìè÷åñêîãî âðåìåíè(TDT).
Ðàçíîñòü ET è UT íàçûâàåòñÿ ΔT. Îíà èçìåíÿåòñÿ íåðåãóëÿðíûì îáðàçîì, íî äîëãîâðåìåííûé òðåíä â ïàðàáîëè÷åñêèé â ñâÿçè ñ ðîñòîì äëèíû ñîëíå÷íûõ ñóòîê â ñðåäíåì íà 1,7 ìñ çà ñòîëåòèå: ýòî óìåíüøàþùàÿñÿ âåëè÷èíà ñ àíòè÷íûõ âðåì¸í äî êîíöà XIX âåêà[20], è óâåëè÷èâàþùàÿñÿ ñ òåõ ïîð (ñì. ñåêóíäà êîîðäèíàöèè).
Ìåæäóíàðîäíîå àòîìíîå âðåìÿ TAI áûëî ïîëîæåíî ðàâíûì UT2 â ìîìåíò 00:00:00 1-ãî ÿíâàðÿ 1958 ãîäà.  ýòîò ìîìåíò ΔT èìåëî çíà÷åíèå îêîëî 32,18 ñåêóíäû. Ïîçäíåå ðàçíèöà ìåæäó ìåæäó Çåìíûì âðåìåíåì TT (ïðååìíèêîì ýôåìåðèäíîãî âðåìåíè) è àòîìíûì âðåìåíåì TAI áûëà îïðåäåëåíà ÷åðåç:
- 1977 ÿíâàðü 1,0003725 TT = 1977 ÿíâàðü 1,0000000 TAI, èëè
- TT â TAI = 32,184 ñ.
Ýòó ðàçíèöó ìîæíî ñ÷èòàòü ïîñòîÿííîé, òàê êàê ñêîðîñòè TT è TAI ïî èõ îïðåäåëåíèþ ñîâïàäàþò.
[ïðàâèòü] Ïðèìå÷àíèÿ
- â‘ Òî÷íîå îïðåäåëåíèå âêëþ÷àÿ èñòîðè÷åñêèé àñïåêò ïðèâåäåíî â 'ESAE 1961': 'Explanatory Supplement (1961), îñîáåííî ñ. 69 è 87.
- â‘ Ô.Ðèëå. Ñòàíäàðòû ÷àñòîòû. Ïðèíöèïû è ïðèëîæåíèÿ. Ì.: Ôèçìàòëèò, 1989, ñ. 384.
- â‘ 1 2 S. Newcomb, "Tables of the Motion of the Earth on its Axis and Around the Sun", in "Tables of the Four Inner Planets", vol. 6, part 1, of Astronomical Papers prepared for the use of the American Ephemeris and Nautical Almanac (1895), at pages 1169.
- â‘ òî åñòü, 31 äåêàáðÿ 1899
- â‘ 1 2 G. M. Clemence (1948).
- â‘ W de Sitter (1927).
- â‘ 1 2 H Spencer Jones (1939).
- â‘ Clemence (1948), at p.171.
- â‘ 1 2 3 4 5 ESAA (1992), see page 79.
- â‘ «âthat in all cases where the mean solar second is unsatisfactory as a unit of time by reason of its variability, the unit adopted should be the sidereal year at 1900.0, that the time reckoned in this unit be designated ephemeris time»
- â‘ Íà Ãåíåðàëüíîé àññàìáååé IAU â Ðèìå â 1952 ãîäó, ñì. ESAE (1961), ðàçäåë 1C, p.9; à òàêæå Clemence (1971).
- â‘ ESAA 1992, p.79: ñî ññûëêîé íà ðåøåíèå Ìåæäóíàðîäíîãî êîìèòåòà ìåð è âåñîâ (CIPM), Ñåíòÿáðü 1954.
- â‘ ESAA (1992), ñì. page 80, ñî ññûëêîé íà ðåêîìåíäàöèþ CIPM îò îêòÿáðÿ 1956 ãîäà, îäîáðåííóþ â 1960 ãîäó Ãåíåðàëüíîé êîíôåðåíöèåé ïî ìåðàì è âåñàì (GCPM).
- â‘ 1 2 3 ESAA (1992), at page 42.
- â‘ 1 2 3 W Markowitz, R G Hall, L Essen, J V L Parry (1958)
- â‘ 1 2 Wm Markowitz (1988).
- â‘ Äëèòåëüíîñòü ñðåäíèõ ñîëíå÷íûõ ñóòîê íå îïðåäåëåíà íà ñ. 9, íî âûðàæåíà íà ñ. 20 Òàáëèö (1895).
- â‘ 1 2 Clemence (1948), p.172, following Spencer Jones (1939).
- â‘ "The origin and rate of ephemeris time are defined to make the Sun's mean longitude agree with Newcomb's expression". ESAE (1961), p. 70.
- â‘ 1 2 L V Morrison & F R Stephenson (2004); also F R Stephenson, L V Morrison (1984), and F R Stephenson, L V Morrison (1995).
- â‘ Clemence (1948), at pp.171-3.
- â‘ W Markowitz & others (1955); W Markowitz (1959); also W Markowitz, R G Hall, L Essen, J V L Parry (1958).
- â‘ 1 2 B Guinot & P K Seidelmann (1988), at p.305.
- â‘ W G Melbourne & others, 1968, ðàçäåë II.E.4-5, ñ. 15-16, âêëþ÷àÿ ñíîñêó 7, îòìåòèë, ÷òî ïðîãðàììû îáðàáîòêè äàííûõ è ñëåæåíèÿ çà êîñìè÷åñêèìè àïïàðàòàìè Ëàáîðàòîðèè ðåàêòèâíîãî äâèæåíèÿ (JPL) ÑØÀ (âêëþ÷àÿ ïðîãðàììó îïðåäåëåíèÿ îðáèòû ñ îäèíàðíîé òî÷íîñòüþ) èñïîëüçîâàëè â êà÷åñòâå ET òåêóùåå çíà÷åíèå àìåðèêàíñêèõ àòîìíûõ ÷àñîâ A.1 ñî ñäâèãîì 32,25 ñåêóíä.  îáñóæäåíèè òàêæå îòìå÷àåòñÿ, ÷òî òàêîå èñïîëüçîâàíèå áûëî "íåàêêóðàòíûì" (óêàçàííîå çíà÷åíèå ñäâèãà íå ñîâïàäàëî íè ñ îäíîé äðóãîé èç ðåàëèçàöèé ET, òàêèõ êàê ET0 è ET1), è õîòÿ A.1 äàâàëè "îïðåäåëåííî ëó÷øåå ïðèáëèæåíèå ê ðàâíîìåðíîìó âðåìåíè ÷åì ET1" íå áûëî îñíîâàíèé ïîëàãàòü, ÷òî àòîìíûå ÷àñû èëè äðóãèå ðåàëèçàöèè ET ÿâëÿþòñÿ (ñîâåðøåííî) îäíîðîäíûìè. Êàê ñëåäóåò èç ðàçäåëà II.F, ñ. 18-19, óëó÷øåííûé ñïîñîá èçìåðåíèÿ âðåìåíè (A.1 + 32.15 ñåêóíä), ïðèìåíÿâøèéñÿ â JPL â ïðîãðàììå îïðåäåëåíèÿ îðáèòû ñ äâîéíîé òî÷íîñòüþ, òàêæå íàçûâàëñÿ ET.
- â‘ G M R Winkler and T C van Flandern (1977).
- â‘ IAU resolutions (1976); see also ESAA (1992) at p.41.
- â‘ ESAA 1992, at p.612.
- â‘ "Improved Lunar Ephemeris", US Government Printing Office, 1954.
[ïðàâèòü] Bibliography
- G M Clemence, "On the System of Astronomical Constants", Astronomical Journal, vol.53(6) (1948), issue #1170, pp 169179.
- G M Clemence (1971), "The Concept of Ephemeris Time", Journal for the History of Astronomy, vol.2 (1971), pp. 7379.
- B Guinot and P K Seidelmann (1988), "Time scales Their history, definition and interpretation", Astronomy and Astrophysics, vol. 194 (nos. 12) (April 1988), pp.304308.
- 'ESAA (1992)': P K Seidelmann (ed.), "Explanatory Supplement to the Astronomical Almanac", University Science Books, CA, 1992 ; ISBN 0-935702-68-7.
- 'ESAE 1961': "Explanatory Supplement to the Astronomical Ephemeris and the American Ephemeris and Nautical Almanac" ('prepared jointly by the Nautical Almanac Offices of the United Kingdom and the United States of America', HMSO, London, 1961).
- IAU resolutions (1976): Resolutions adopted by the IAU in 1976 at Grenoble.
- "Improved Lunar Ephemeris", US Government Printing Office, 1954.
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